Chapter II: Crystalline Rocks

Distribution of the crystalline rocks. Typical traverses. The harder and softer gneisses. The Jebba quartzites: the phyllites and slaty schists of the Niger valley; the sheared trachytes of Bussa; the Ngashki phyllites; replacement of the softer gneisses by the harder banded types. The gneisses and schists of Ilorin; the quartzites of the Orissa hills; the stanniferous pegmatites of Eri. The muscovite schists of Northern Kabba; the quartzites of Lukke; the crystalline limestone between Jakura and Wa; the mica schists of Sequarre; the magnetite and hæmatite schists of Eastern Kabba; the Igbo limestone. The phyllites and schists of Sokoto province; the volcanic agglomerate of Maradu; the blue quartzite of Kazaure; the diorites of Kano; Mounts Dala and Kogon Dutsi; the gneisses and schists of north-eastern Kontagora; the Zungeru belt of phyllites and schists; auriferous quartz reefs; the quartzites and schists of Eastern Zaria; the gneisses and granites of Abuja; the tourmaline rocks of Darroro; the mica schists of Gwombe; the banded gneisses of Bauchi province; the granites of the Murchison Hills; the later intrusions. The inliers of Arofu; the granites and gneisses of southern Muri; the Vere Hills; exfoliation in the Kilba Hills; the crystalline rocks of Bornu.

While crystalline rocks undoubtedly everywhere form the foundation of the Protectorate, there are extensive tracts over which the later sedimentary rocks obscure the underlying granites, gneisses, and schists. The largest area over which these crystalline rocks are exposed is that which occupies the centre of the Protectorate and extends from the Niger in the west to beyond Bauchi in the east, and from Nassarawa in the south to the neighbourhood of Katsina in the north. The major portion of the provinces of Kabba, Ilorin and Borgu to the west of the Niger also possess a crystalline floor, while similar rocks cover a narrow strip on the southern border of Muri and Yola, as well as a considerable tract of country to the east of the Gongola, lying partly in Yola and partly in Bornu.

_Typical Traverses._

The following typical traverses show the character of the exposures along the tracks.

I. Filelin (near Lokoja) to Ohoto:

9.20 A.M.— Left Filelin.

9.34 „ Blocks of ferruginous sandstone and grit.

9.38 „ Masses of biotite pegmatite; large felspars often
graphic.

9.40 „ Fine-grained, non-foliated reddish biotite granite with
streaks and veins of graphic biotite pegmatite. The
granite at places on the margin follows the foliation of
the adjoining biotite gneisses, in places crosses the
foliation, or is in part injected into the gneiss. The
biotite gneiss is often folded and distorted; in places
there is an alternation of dark bands of biotite gneiss
with lighter coloured felspathic layers. The foliation
is much disturbed, but in a general way E.N.E. and
vertical. The gneiss is cut by granite and pegmatite
veins and dykes.

9.45 „ Reddish granite continued. In places the granite shows
distinct banding and alternation of coarse and fine
layers; sometimes banded with pegmatites.

9.50 „ Same pink granite; but here an irregular mixture of
coarse and fine material with in places knots of coarse
granite in a fine grained matrix.

10.0 „ Small granite knobs on right.

10.3 „ Sandstones.

10.10 „ Biotite gneiss, strike N.E.–S.W., foliation dip 40° N.,
cut by coarse pegmatites as before; in places
interfoliated bands of segregation pegmatite.

10.15 „ Soft biotite gneisses.

10.30 „ Large blocks of pink granite and pegmatite.

10.40 „ Granite piercing hard white biotite gneiss with knots of
amphibolite. Strike of gneisses N. 35° E., foliation dip
N.W. There is a sharp junction between the granite and
gneiss, but no chilled margin.

10.50 „ Granite.

11.0 „ No exposures.

11.30 „ Extensive mass of coarse-grained biotite gneiss, crossed
by pegmatite veins and dykes of coarse red granite with
large felspars.

11.35 „ Biotite augen-gneiss with felspars sometimes two inches
long; crossed by veins of fine red granite or aplite.

11.40 „ No exposures.

12.30 P.M.— Bare surface of augen-gneiss level with the plain.

12.40 „ Dyke or sheet of fine grained reddish granite showing
distinct foliation.

12.45 „ Fine grained and finely banded biotite gneisses crossed
and injected by granite veins.

12.50 „ Sandstones.

1.15 „ Ohoto.

II. Jakura to Wa (north side of Mimi Valley).

6.30 A.M.— Left Jakura hill; on brow of hill quartz schist, strike
N. 10° W., foliation vertical; at foot of hill fine
grained pink granite with pegmatite, passing into
friable pink granite with large crystals of graphic
quartz and felspar in a finer matrix.

6.55 „ Soft biotite gneisses, strike N.N.E., dip W.N.W.

6.58 „ Band of amphibolite, dip unascertainable, followed by
soft biotite gneisses. Pebbles of hornblende epidote
schist.

7.12 „ Bare exposure of red pegmatite dyke cutting fine grained
felspathic gneiss, with in places a sharp junction and
in others not.

7.20 „ Quartz schist.

7.35 „ Magnetite hornblende schist, strike N. 10° W., foliation
dip practically horizontal.

7.40 „ Quartz hæmatite schist, dip unascertainable.

8.0 „ Massive and granular quartzite and quartz schist.

8.15 „ Biotite gneiss.

8.30 „ Quartz-muscovite schist, strike N.N.E., dip low to
W.N.W.

8.35 „ Pebbly muscovite schist, strike N.–S., foliation
vertical or highly inclined.

8.45 „ Pegmatites.

8.50 „ White marble forming rounded knobby hills cut by
pegmatite veins and enclosing streaks and veins of
quartz-epidote-calcite-idocrase rock on right hand; on
left the marble is foliated, with folia of pyrites and
knots of quartz; strike of foliation N. 10° E., dip 10°
W.

9.5 „ Marble ends.

9.8 „ No exposures; grass land.

9.15 „ Quartzites.

9-35 „ Soft biotite gneiss; foliation generally vertical;
strike N.N.E., dip vertical or high towards W.

9.50 „ Garnetiferous biotite gneiss with quartz segregations;
folia crumpled and contorted; strike generally N.–S.

10.10 „ Garnetiferous biotite gneiss as before; folia
practically horizontal.

10.25 „ Soft micaceous schists pierced by granite; strike N. 35°
E., dip S.E.

10.35 „ Pink granite in places foliated; graphic pegmatites on
slope of Wa hill.

10.45 „ Wa. On top of hill white mica schists with magnetite
knots, strike N.–S., dip 20° W.; cut by
quartz-muscovite-magnetite pegmatites.

III. Olle to Sura (Central Kabba).

6.20 A.M.— Left Olle.

6.25 „ Quartz schists and quartz-muscovite schists, strike N.
40° E., dip 55° W.

6.27 „ Soft fine-grained quartz-muscovite schists, strike N.
10° E., dip 70° W.

6.35 „ Muscovite schists, strike N. 30° W., foliation vertical.

6.40 „ Thick dyke of graphic quartz and felspar, with crystals
of tourmaline.

7.5 „ Fine grained white granite.

7.25 „ Coarse white granite in river bed.

7.30 „ No exposures.

8.10 „ Micaceous gneiss, strike N. 35° E.; much folded and
contorted with streaks and knots of pegmatite, probably
proceeding from a thick intrusive mass of quartz-felspar
pegmatite which adjoins the gneiss.

8.35 „ Muscovite schists, strike N.–S., foliation vertical.

8.40 „ Track leads along valley between hills of white
quartzite; loose blocks of compact and granular
quartzite.

9.0 „ Summit of pass; blocks of white quartzite.

9.5 „ Muscovite schists, strike N.–S., foliation vertical.

9.20 „ Lukke.

9.30 „ Soft muscovite schists and quartzites.

9.40 „ Muscovite schists, strike N. 10° W., foliation vertical.

9.50 „ Broken fragments of quartzite, quartz-muscovite
pegmatite, and quartz-tourmaline pegmatite on road;
intrusive masses of graphic felspar and quartz with
crystals of muscovite, associated with exposures of
coarse muscovite granite.

10.10 „ Biotite gneisses much crumpled and folded, exposed in
bed of stream, strike N.W.

10.15 „ Quartzites and muscovite pegmatites.

10.20 „ Intrusive mass of muscovite granite with large crystals
of graphic quartz and felspar in a fine grained granitic
matrix; banded in places with coarse and fine layers.

10.40 „ Exposures of quartz-muscovite pegmatite.

11.0 „ Soft micaceous schist, strike N. 10° W., foliation
vertical.

11.5 „ In bed of river and for 500 yards beyond, garnetiferous
mica schists with quartz lenticles, strike N. 10° W.,
dip 15° W.

11.30 „ Schistose biotite granite, with schistose pegmatite
veins forming Sura hill.

11.40 „ Sura.

IV. Agwede to Kabba.

6.30 A.M.— Left Agwede hill. On top, white granite usually showing
faint foliation and frequently enclosing portions of
dark mica schist; on slope, whitish ribboned gneisses,
often folded and contorted, strike N.E. to E., foliation
vertical.

6.40 „ Dyke of graphic pegmatite in gneiss; sharp junction.
Frequent bands of schistose pegmatite in the pale
coloured banded gneisses; in places the gneisses are
hornblendic with knots of fine-grained hornblende gneiss
or schist.

7.0 „ Ascent to plateau over contorted gneisses.

7.20 „ Same contorted gneisses passing into and invaded by
white schistose granite; wooded plain, few exposures.

8.20 „ Occasional exposures of similar contorted and granitised
gneiss; strike E. 10° N. to E. 10° S.

9.5 „ Faintly banded and indistinctly foliated biotite gneiss.

9.30 „ Odouape.

10.30 „ Level surface of composite gneiss.

11.30 „ Much crumpled and folded biotite gneiss; strike N.–S.
Followed by garnetiferous biotite gneiss.

11.45 „ Composite gneiss; foliation vertical; strike N. 10° W.

12.0 „ Kabba.

V. Izon to Abuja (Nassarawa province).

6.25 A.M.— Left Izon. Open grassy or thinly-wooded plains for two
hours; no exposures.

8.20 „ Hills; ascent of 300 feet; biotite and hornblende gneiss
and amphibolite; epidote-hornblende gneisses; in places
twisted and contorted and extensively invaded and
injected by masses of biotite granite; strike varies
between N. 10° E. and N. 10° W.; foliation mostly
vertical.

9.0 „ Open plain; thick granite wash.

9.30 „ Hills; granite invading hornblende epidote gneisses.

9.42 „ Abuchi. Amphibolite veined with granite.

10.0 „ Narrow plain; thick drift.

10.30 „ Hills; soft micaceous gneisses, strike N. 10° W.,
foliation vertical or high to E.

10.35 „ Hard biotite gneisses, strike N.–S., foliation linear
and vertical.

10.45 „ Ribboned and banded biotite gneisses with quartz veins
parallel to or crossing the foliation; strike N.–S.;
foliation vertical.

11.0 „ Abuja granite with foliated margin; vertical crush lines
in granite striking N.–S.

11.15 „ Abuja Residency.

VI. Boi to Ronn (Southern Bauchi).

6.30 A.M.— Left Boi; hornblende epidote gneisses broken and invaded
by granite; in places much folded and crumpled.

6.40 „ Felspathic gneiss, strike E.–W., dip high to S.

7.20 „ Complex of quartz epidote rock, amphibolite, epidote
hornblende schist, and gneiss, in places much
granitised; general strike E. 10° N.

7.45 „ Hummocks of banded gneiss; open country; few exposures.

8.40 „ Hornblende epidote gneisses.

8.45 „ Goshin Duchi. Hills of biotite granite behind town.

9.3 „ Quartz schists, fine grained gneisses, epidote
hornblende and quartz hornblende rocks, much granitised
in places veins and dykes of pegmatite and basalt.

9.20 „ Blocks of basalt and porphyry.

9.35 „ Hills of fine grained biotite granite.

10.15 „ Granite rocks, in places schistose.

10.45 „ Rocky, hilly country; banded and contorted gneisses
micaceous and hornblendic; general strike E. 10° S.

11.10 „ Ronn.

It is unnecessary, however, to quote the details of particular traverses at greater length, especially as such descriptions serve only to show the character of the phenomena to be correlated. It is sufficiently evident that there are within the Protectorate two fairly well-defined series of crystalline rocks, a less metamorphosed series of quartzites, schists and gneisses representing an original sedimentary succession, and a more highly metamorphosed series of banded and contorted gneisses, whose relation to the less metamorphosed group will be discussed in the sequel. The less metamorphosed gneisses present in the field somewhat granular and incoherent outcrops and may be conveniently termed the “softer” gneisses, while the “harder” or more highly metamorphosed gneisses are readily distinguishable by the smooth and compact character of their exposed surfaces. The softer gneisses appear to disintegrate readily into a gritty sand under present conditions, while the harder gneisses exfoliate and pass into angular débris. Both series are extensively invaded by granitic and other igneous intrusions.

_Borgu and the Niger Valley between Jebba and Yelwa._

In the province of Borgu the superficial accumulations are so extensive and the exposures so few and so widely scattered that the general character of the underlying rocks is somewhat difficult to ascertain. It is probable, however, that, like the neighbouring region of Nikki,[18] there is a floor of well foliated and granitoid gneisses of varied origin, much pierced and invaded by veins of granite, pegmatite, quartz porphyry and quartz. Tremolite schists are known from Mori, and sheared porphyries from Kanikoko. The occasional inselberge are composed invariably of grey porphyritic granite. At Jebba the gorge of the Niger is carved out of a series of sheared quartzites and pebbly grits with the shear planes at times well spaced and the deformed pebbles still distinctly visible. Secondary mica has been sparingly formed along the planes of movement, together with knots and streaks of tourmaline and tourmaline quartz. The predominant strike is N.–S., and the prevailing dip of the foliation 65° W. The well-jointed Juju Rock is composed of a laminated quartzite schist, which splits readily into thin curving slabs, whose surfaces are coated continuously with white mica and show a well-defined strain-slip corrugation. Westward the Jebba quartzites pass into highly sheared quartz schists, quartz mica schists and granulitic gneisses, which are followed by crumpled and contorted muscovite biotite schists and gneisses and by granite at the river Awo. The quartzites and granulitic gneisses are produced along the strike to form the range of low wooded hills which stretches southward from Jebba towards Ilorin. To the east of Jebba the quartzites are abruptly limited by hard banded and composite gneisses pierced by masses of porphyritic biotite granite.

To the north of Jebba the Kailema granite, over which the track leads from Jebba to Bajibo, is a massive hornblende granite, porphyritic with white and pink felspars and somewhat finer-grained on the margin. The granite has evidently pierced a series of hornblende, epidote and talc schists and micaceous gneisses which outcrop imperfectly to the east. Still farther north, at Wuru, the channel of the Niger is occupied by a hard and coarsely foliated epidote hornblende gneiss, much pierced by irregular veins of schistose granite and pegmatite. The foliation planes of the gneiss are vertical and strike approximately N. 20° E. On the left bank the gneiss is succeeded by thinly bedded quartzites and quartz schists with scattered knots, lenticles and veins of quartz and finely fibrous tourmaline. Intercalations of much corrugated and wrinkled mica schist bear witness to differential movements along certain lines. The whole series, which strikes N.–S. and dips vertically or at high angles to the east or west, has apparently suffered a rapid and repeated folding. These rocks are followed by a series of fine-grained green and yellow quartzites and well-cleaved slaty schists with numerous quartz reefs, which pass eastward into a complex of hornblende gneisses, epidote hornblende gneisses, amphibolites and fine grained schistose granites which disappear at Kumboji under the sandstones of southern Kontagora.

At Bussa on the right bank of the Niger, near the Residency, there are exposures of much decomposed shaly phyllites, vertical or highly inclined to the east and striking N. 10° E. These are followed on the left bank by a thick series of little altered grey and greenish granular quartzites, which in places become pebbly and conglomeratic and enclose bands of greenish trachyte, also showing little or no sign of shearing. Further exposures of black and coarsely-sheared trachytes are found in the swamps between Bussa and Libata. Eastward the quartzites are much invaded by masses of dark green diorite and by thick veins of tourmaline quartz, the latter showing every variation between a banded tourmaline quartz rock and a felted mass of finely fibrous tourmaline. Nearer Auna these rocks are succeeded by well-foliated augen gneisses which are overlaid by the sandstones of southern Kontagora.

To the north of Bussa phyllites are again exposed in the low rounded hills to the north-west of Ngashki, where they strike N.–S., and possess a high dip of 65° to the east. Locally, however, the strike may be directed as much as 35° to the east of north, indicating that the rocks do not pursue a constantly rectilineal direction from south to north, but rather possess a zig-zag character with short and sharp deflections towards the east. The phyllites, which are mostly grey in colour, but in places mottled with large irregular whitish spots on a grey ground, pass eastward into soft grey sericitic schists with crystals of magnetite and pyrites. The whole series is pierced by quartz tourmaline veins, in which all proportions are to be found, from almost pure quartz through banded quartz tourmaline rocks to masses of pure tourmaline of the finely fibrous variety. Sills of much decomposed vesicular dolerite are common also amongst the phyllites. Farther east intercalations of micaceous gneiss become abundant within the schists, and a red fine- grained granulitic gneiss intervenes between them and the massive biotite granite of Abara, with its large crystals of felspar. To the north of Ngashki, phyllites and green slaty schists with lenticular streaks and veins of quartz and tourmaline quartz occur near Lokofe. A good exposure in the river Malenda shows pebbly lustrous schists striking N.–S., and dipping steeply towards the east. At Yelwa a narrow strip of phyllites and slaty schists on the left bank of the Niger, passes westward into knotted mica schists and biotite gneisses in the river bed and eastward into the granitic gneiss of Bin Yauri. In the country which lies to the east of Lokofe and Yelwa, and especially in the neighbourhood of Tabachi and Maibirro, green slaty schists of varying hardness are frequently exposed, usually associated with harder gritty bands and much invaded by veins of quartz and tourmaline quartz, and irregular intrusions of granite. In places also there may be found intercalations of hard, gritty, carbonaceous and little-metamorphosed slaty beds. At Chuggu, moreover, between Tabachi and Maibirro, there is a parallel belt of biotite gneisses, hornblende gneisses, and epidote hornblende gneisses, while at Maibirro the slaty schists, here associated with thinly-bedded quartzites, quartz schists and quartz muscovite schists, striking N. 35° E., with a dip to the north-west of 65°, are succeeded in the river Malenda by well-banded biotite gneisses much veined with felspathic and granitic material. The actual junction of the banded gneisses with the quartz schists is, however, not exposed.

The rocks of the Niger valley, between Jebba and Yelwa, are thus of many and varied types. The quartzites and phyllites, the slaty schists and micaceous gneisses, and the green hornblendic and epidotic beds represent the sandstones, shales, grits and arkoses of the sedimentary series, while the sheared trachytes and porphyries, the schistose granites and the coarse-grained amphibolites represent the original igneous intrusions within the sedimentary succession. The whole series strikes fairly constantly N.–S., and forms a practically continuous belt between Jebba and Yelwa. In most places there are indications that the rocks have suffered a repeated folding into fairly sharp anticlines and synclines with vertical axial planes, the folding being apparently more rapid in the phyllites and schists than in the quartzites and gneisses. There has unfortunately been no opportunity of tracing the belt in detail along the strike, and little is known of the lateral variation or repetition of the individual members or of the original sequence of the sedimentary succession. There is no reason to believe that there has been any increase or decrease in the intensity of the metamorphism throughout the belt from south to north, and no indication has been found of the occurrence of any thermal metamorphism in the neighbourhood of the later granitic intrusions.

The most remarkable feature, however, about this belt of schistose rock within the Niger valley is the manner in which the less metamorphosed series of quartzites, phyllites and schists is bounded abruptly and locally replaced by a more highly metamorphosed and much granitised series of well-foliated and often banded gneisses. At Maibirro, for example, the schists are abruptly succeeded by banded gneisses of a type which is nowhere found between Maibirro and the Niger. Similarly at Wuru and Jebba, the quartzites give place unexpectedly to composite granitised gneisses, which must have originated under very different conditions from those which brought about the metamorphism of the phyllites and schists. This is a peculiarity of the fundamental architecture which is repeated in many other parts of the Protectorate.

_Kabba and Ilorin._

Over the greater part of the province of Ilorin the surface accumulations are so extensive that the nature of the underlying rocks can be judged only from small and scattered exposures. So far as known there are no belts of phyllites and quartzites upon the western or central plains comparable with that of the Niger valley above Jebba. The predominant rock in central Ilorin is a fairly coarse-grained, granular and felspathic muscovite biotite gneiss, sometimes an augen-gneiss with large eyes of felspar, which disintegrates readily into a felspathic sand. With it are associated in places thin bands of quartzite, quartz schist, quartz muscovite schist, mica schist and gneiss, often garnetiferous, amphibolite and schistose granite, which, however, are neither sufficiently extensive nor sufficiently resistant to give rise to any prominent feature upon the surface. The whole series which strikes approximately N.–S. is much veined by muscovite and tourmaline pegmatites and invaded by masses of granite usually of a more or less foliated character. The larger masses rise into turtlebacks or heaps of boulders, and in their neighbourhood the gneisses have often been so intimately veined and traversed by the granite as to produce a composite or banded rock. At Share hard biotite gneisses with linear foliation are extensively developed and associated with hornblendic gneisses and amphibolites and cut by tourmaline pegmatites.

In the Orissa hills, which lie to the east of the great plain of Ilorin, the softer gneisses enclose for the first time a prominent series of quartzites, quartz schists, quartz mica schists and granulitic gneisses which, with their general meridional strike and high dip to the west, are very similar to those which form the low hilly ground to the south and west of Jebba. The Shappa hills which succeed the Orissa hills to the south are composed in great part of biotite and hornblende granites, coarse-grained, fine-grained and porphyritic, which appear to be intrusive into a series of hard biotite and hornblende gneisses, in places well-banded and much veined with pegmatites, which occupies the south-eastern corner of the province. Around Eri the softer gneisses are again well developed and extensively exposed, the series here consisting of thin quartzites, felspathic gneisses and much contorted and occasionally garnetiferous micaceous gneisses and schists, with interbedded hornblende schists and irregular masses of coarse-grained amphibolite, probably of intrusive origin. The whole series is much pierced by veins of quartz, tourmaline quartz and muscovite pegmatite, the latter in places carrying tinstone.

PLATE VI

The softer gneisses and schists are found everywhere in northern Kabba between Eri and the Ilorin border and the sandstone plateaux which fringe the Niger. The usual succession of thinly bedded micaceous and felspathic gneisses and hornblende schists is varied by the intercalation of several belts of platy muscovite schists and quartz muscovite schists, whose deeply decomposed surface usually supports a luxuriant forest growth. Within the area mentioned there are at least four of these belts, more or less well-defined and continuous along their strike from north to south. The first runs southward from the neighbourhood of Idofin by way of Kponyon, Onore, Ote, and Omwo into southern Kabba: the second strikes southward from Okoro and Iye, skirting Sanlu to the east and Moppa to the west: the third runs from Shale and Oferre by way of Lukke, Olle, and Appe to the neighbourhood of Odouape, and the fourth is exposed along the track leading from Wa to Jakura on the north side of the Mimi valley. While the fissile quartz muscovite schist may be taken as the index rock of the various belts, each belt is in itself a complex of muscovite schists, quartz muscovite schists, quartz schists, quartzites, biotitic and hornblendic schists and other rocks and the proportions of these rocks vary not only in the successive belts but at different points within the same belt. In the third belt for example there is along with the muscovite schist a great development of quartzites and quartz schists around Lukke, where they form prominent rocky hills (_v._ Traverse No. III. p. 66). Farther north several bands of chlorite and talc schist are found in the same belt in the neighbourhood of Iye, while in the fourth belt between Wa and Jakura there are intercalations of such other rocks as quartz magnetite schist, magnetite hornblende schist, and crystalline limestone (_v._ Traverse No. II. p. 65). The limestone is a white coarse-grained saccharoid marble which may be traced for at least 1,000 feet across the strike without affording any indication of repetition by folding. On the margin the marble is distinctly schistose with the foliation planes striking N. 10° E. and dipping at a low angle towards the west. The rocks in the neighbourhood are somewhat irregularly folded with the foliation planes as a rule parallel with the bedding. Their junction with the marble is not exposed, but if the marginal foliation of the latter be also parallel with the bedding, the actual thickness of the original limestone must have been very much less than 1000 feet. The marble is very pure, consisting almost entirely of calcite with occasional streaks and segregation bands of quartz-epidote-calcite-idocrase rock representing the original impurities within the limestone. Minute spots of graphite are abundant throughout and on the margin streaks and knots of quartz and pyrites are common along the foliation planes. In places the marble is irregularly crossed by later pegmatite veins.

The tracts which intervene between the various belts are occupied by a variety of soft micaceous and hornblendic schists and differ from the belts already described only in the comparative rarity of muscovite schists and quartzites. The absence of sufficiently detailed investigation, however, makes it impossible to say to what extent the various belts described above represent repetitions of one and the same belt, or to what extent they represent the original sequence of the sedimentary succession. While the average strike of the whole series is meridional, its undulating or zigzag character is evident from the occurrence of individual exposures showing every angle between north and east. The different members of the series, while frequently vertical, may dip at various angles to the east or west or even in places approach the horizontal. Individual bands are frequently puckered and contorted and the whole series is invaded in places by extensive masses of biotite granite which rise into turtlebacks and groups of rounded hills. The granites are usually foliated on the margin and sometimes throughout and frequently show at their junction with the gneisses and schists a narrow zone of injected rock. Small detached exposures of the harder banded gneisses, elongated upon a parallel strike, are distributed irregularly throughout the softer series. The gneisses and schists are abundantly pierced by muscovite and tourmaline pegmatites and veins of quartz and tourmaline quartz, the latter in the neighbourhood of Iye being minutely banded as in the case of some of the tourmaline quartz veins at Bussa and Ngashki. With the pegmatites appear to be connected some small intrusive masses of non-foliated granite which become more numerous in the eastern portion of the area under consideration (_v._ Traverse No. I. p. 64).

The striking feature of the architecture of the high plains of central Kabba is the manner in which the softer gneisses of the north with their intercalated bands of schists and quartzes are invaded, and in part replaced, by masses of granite and of hard banded and composite gneisses. The first belt of schists can be traced southward with intervals from Omwo to the neighbourhood of Arigidi and Sequarre, where the soft micaceous schists carry much staurolite. Harder gneisses however appear to bound this softer belt irregularly to the west and east. The prolongation of the second belt of schists in northern Kabba is probably represented by two narrow strips of quartzite enclosed in the harder gneisses between Kabba and Lupa. The third belt appears to be somewhat abruptly replaced in the neighbourhood of Odouape by a complex mass of granites and hard composite and banded gneisses which extends westward across the high plains of Kabba from Agwede and Shokko Shokko to Ikinre and Omwo (_v._ Traverse No. IV., p. 67), and is apparently prolonged southward into the Egbira hills. The fourth belt appears to suffer much lateral variation, and is represented on the main road between Lokoja and Kabba only by some thin bands of muscovite schist enclosed within a series of micaceous and felspathic gneisses.

In southern Kabba the few facts at our disposal seem to indicate the almost complete absence of quartzites and muscovite schists and a repeated alternation of the softer and harder gneisses, the whole series being pierced by pegmatite veins and intrusive masses of granite. Quartzites and quartz schists reappear, however, in the Kukuruku hills on the borders of Northern and Southern Nigeria.[19]

The lower plains which stretch eastward to the Niger from the escarpments of the Egbira and Agwede hills are occupied very largely by gneisses of the softer series. Quartzites, quartz schists and quartz- muscovite schists occur within the series, but their distribution over the plains is so irregular, and their prolongation along the strike so inconstant, that they cannot in any way be correlated with the quartzites and schists of either the third or fourth belts in northern Kabba. It would seem, indeed, that instead of the regular alternation to be found in the north, there is over these eastern plains a greater mixing of types and a greater variation in character of the individual members when traced along the strike. Quartz-magnetite and quartz- hæmatite schists are fairly common, typical occurrences being found in the neighbourhood of Bangede and around Okuruku and Anamu near Okeli, and on the plains to the west of the Againyi hills, as well as at the foot of their western slope. Amongst the granites and granitoid gneisses which form the escarpment of the Egbira hills, bands of felspathic gneiss and micaceous schist are frequently enclosed, and on the plains the gneisses and schists are crossed in the usual manner by pegmatite veins and pierced by masses of more or less foliated granite, which form the prominent isolated hills which diversify the surface of the plain. Here and there also may be found a small exposure of composite or banded gneiss, but it is only in the immediate neighbourhood of the Niger that the harder gneisses attain any extensive development. There they form a narrow belt, marked by prominent intrusions of granite, which stretches southward from Lokoja along the right bank of the river, and adds picturesqueness to the Niger valley between Igbo and Itobe (p. 6). At Lokoja the harder gneisses are exposed in the river bed, and the softer gneisses and schists at the foot of Mt. Patti. The average strike is N.E. to S.W., and the whole series is much pierced by veins of granite and graphic pegmatite, the latter containing frequent knots of magnetite.

To the east of the Niger the softer gneisses reappear in the narrow strip of crystalline rocks in Bassa province which fringes the left bank of the river. Typical exposures are found in the neighbourhood of Igbo and Itobe, where the gneisses are associated as usual with early foliated or partially foliated granites and with later intrusions of muscovite and tourmaline granites and pegmatites, the tourmaline occurring as a rule in large and well-formed crystals. At Igbo the micaceous gneisses enclose a number of narrow bands of fine-grained, crystalline limestone, each 3 to 4 feet in thickness. The limestone varies much in colour and hardness, the purest varieties being blue, while weathering and impurities produce grey and brown tints. Much of the limestone is gritty, but the commonest accessory is a pale-coloured tremolite in rosettes and felted veins of fibrous crystals and in scattered well-formed prisms.

The crystalline rocks of Kabba and Ilorin thus present considerable resemblances in individual types to those of the Niger valley above Jebba. The most striking differences are the occurrence of such new types as crystalline limestone and magnetite and hæmatite schist in eastern Kabba and Bassa, and the apparent absence in the southern provinces of the characteristic phyllites and slaty schists of the Niger valley to the north. The place of the phyllites, however, is probably taken by the muscovite and quartz-muscovite schists, which form such prominent members of the metamorphic series in northern Kabba. If so, there would seem to have been a slight increase in the intensity of the regional metamorphism in these southern provinces as compared with the Niger valley above Jebba. Of thermal metamorphism, on the other hand, there is little trace. The schists and gneisses in the neighbourhood of Sequarre in western Kabba and in several localities in northern Kabba and Ilorin contain a considerable quantity of staurolite and other metamorphic minerals. Their distribution, however, appears to possess a linear character which is somewhat inconsistent with a contact origin, while sufficiently extensive igneous intrusions, to whose influence their local development might be ascribed, have nowhere been definitely recognised in the vicinity. The numerous intrusive masses of both foliated and unfoliated granite throughout these provinces appear to have induced little or no contact metamorphism in the surrounding gneisses and schists. An exception to this rule may perhaps be made in the case of the tourmaline granite behind Igbo on the Niger, to whose influence may possibly be ascribed the extensive development of tremolite in the neighbouring crystalline limestones. At the same time it should be remembered that the occurrence of tremolite in metamorphic limestones cannot be taken as an invariable indication of contact action.

The irregular association of a less metamorphosed series of quartzites, schists, and gneisses with a more highly metamorphosed series of well- foliated, banded and contorted gneisses, exhibited by the crystalline rocks of the Niger valley above Jebba, forms likewise a striking feature of the crystalline rocks of Kabba and Ilorin. The banded gneisses may be either biotitic or hornblendic and the lighter and darker bands may alternate so regularly and rapidly as to give rise to a striped or ribboned appearance even in the hand specimen. They are for the most part of a medium and even grain and never porphyritic, although sometimes they enclose lenticular masses of finer grained biotite gneiss or of amphibolite. As a rule they are much invaded by granite and frequently rendered composite by thin granitic intrusions. The foliation, which may be parallel or oblique to the striation, is their least conspicuous feature. Pegmatite veins are usually abundant and frequently foliated. As already described (p. 74) these hard banded and contorted gneisses are found irregularly replacing the softer gneisses both along and across the strike.

So far as known there are no schistose rocks in Kabba and Ilorin comparable to the sheared trachytes of the Niger valley and the sheared porphyries of Borgu. Rare occurrences of thin basaltic veins have been observed, but as a rule the later intrusions in Kabba and Ilorin take the form of small masses of red or white aplitic granite or of dykes and veins of muscovite and tourmaline pegmatite or of graphic granite. These later granites have usually a peculiar granular and incoherent character and never possess chilled margins. Frequent occurrences have been noted of the granular granite acting as a kind of ground mass to large scattered porphyritic masses of graphic quartz and felspar (_v._ Traverses Nos. I, II, III, pp. 64–66). Frequently also the granites show an ill-defined banding caused by an alternation of coarser and finer grained layers. It is possible that the muscovite and tourmaline pegmatites, the veins of graphic granite, the quartz reefs, the banded tourmaline quartz rocks, and the small intrusive masses of acid granite may all possess a certain genetic connection. Only at Igbo, however, does there occur an extensive mass of reddish muscovite tourmaline granite which to some extent unites the characters of all the dyke rocks and represents in a general way the original magma from which the various dykes and veins and intrusive masses have been differentiated. The abundance of tourmaline throughout Kabba and Ilorin led to the hope that tinstone would in places be associated with it, but only at Eri in eastern Ilorin have the pegmatites proved stanniferous. The quartz reefs also have nowhere been found to yield more than the minutest trace of gold.

4. _The Central Crystalline Area._

The peculiarities of structure which have been remarked in the western provinces are continued throughout the larger crystalline area in the centre of the Protectorate. To the east as to the west of the Niger there has been an irregular and local replacement of a series of softer gneisses, quartzites, and schists by a complex of harder composite and often well-banded gneisses. There is also in the western part of the central area within the softer gneissic series a well-defined grouping of the quartzites and phyllites or micaceous schists into regular and definite belts which preserve their identity for considerable distances along the strike. In the east, however, this regular grouping appears to be absent, the quartzites and mica schists being distributed irregularly and in short and narrow bands amongst the felspathic and micaceous gneisses.

In the east of Sokoto province there are at least two broad and well- defined belts of phyllites and indications of a third. The first is found between Bukwium and Anka near the Kontagora border, and is well exposed in the Anka river. The strike of the phyllites varies from N.–S. to N. 20° E. Surface creep here and there makes the dips uncertain, but the inclination is usually high, and is sometimes to the west, sometimes to the east. This fact probably accounts for the great width of the belt. It is not known whether the phyllites extend to the east of Anka, but after crossing them at right angles to the strike for thirty miles to Bukwium they are still in full force, and the western limit is unknown. There are numerous quartzite bands, especially about Anka and to the south of it. The phyllites are soft, fine-grained and dark- coloured, and are traversed by abundant quartz veins, some of which carry pyrites. It is interesting to note that the phyllites are only exposed in the neighbourhood of the river. Away from the river, on the level plain, there is no indication of them, and this fact is of significance when considering the possible extension of the phyllite belts. There is a numerous and interesting series of igneous rocks within the phyllite area, including such types as hornblende granite and syenite, pegmatite and quartz porphyry. These rocks, as a rule, show no sign of crushing, and are probably later intrusions. Some of the porphyries, however, show an imperfect foliation, and appear to have been exposed to some of the later crustal movements. Bands of amphibolite and veins of tourmaline quartz also occur within the phyllitic series. (A. L.)

The second group of phyllites and quartzites forms a belt of low hilly country striking N.N.E.–S.S.W. across the Sokoto-Kano caravan route between the villages of Maradu and Karakai, and continues northward at least as far as Bazai. The series attains across the strike a breadth of about thirteen miles, and consists of a succession of slaty schists and knotted phyllites and quartzites, much pierced by quartz veins and thin bands of schistose granite, and dipping fairly constantly in the west at 70° E.S.E., but gradually heeling over in the east to the W.N.W. Towards Karakai the quartzites predominate and contain much tourmaline, and many samples may be found showing quartz and tourmaline or quartz, slate and tourmaline in alternating ribbon-like bands. The third belt is very imperfectly exposed on the borders of the province between Woneka and Duru, and is represented only by some scattered outcrops of slaty phyllite and mica schist.

Between Maradu and the margin of the Sokoto sandstones to the west, the softer gneisses are exposed upon the sandy plains in a very fragmentary manner, and appear to consist of micaceous and hornblendic gneisses and schists, and of hornblende epidote gneisses and amphibolites with numerous small granitic intrusions. The rocky knobs and kopjes two miles west of Maradu are formed of a coarsely banded volcanic agglomerate, dipping eastward at angles of 30°–40°. The agglomerate may be traced to within a mile of the village, where it gives place to a thinly bedded series of red ashy sandstones and ash beds. East of the rest camp, these beds strike N.N.E. and dip at 40° E.S.E. while west of the camp they strike E.N.E. and dip at 30° S.S.E. The ash is exposed in, and to the west of, the village, but to the east its place is taken by a much broken and shattered granitic rock. The actual junction, however, was not observed. Such evidence as there is points to the former existence of a volcanic neck or vent in this locality.

Between Karakai and Woneka the surface accumulations obscure the rock for long distances, especially to the west and east of the granite hills of Kotorkoshi. Around Bungudu, however, there are numerous exposures of a coarse-grained composite gneiss which probably extends eastward to the boundary of the province. Beyond Duru a grey biotite granite with a distinctly foliated margin forms the broken hilly country between Runka and Kurefi, but between Kurefi and Sani the granite gives place to a group of dark-coloured biotite gneisses before the crystalline rocks disappear under the sandstones of Katsina. Knobs and hummocks of granite, however, protrude through the sedimentary rocks round the walls of the town, and Barth[20] has reported the occurrence of similar mounds and ridges immediately to the north. To the south-east of Katsina the surface rock appears to be everywhere a reddish granite as far as Duru, but beyond Duru the superficial accumulations so effectually cover up all but the occasional kopjes and boulders of granite that the character of the underlying rocks is difficult to ascertain. From the scattered fragments of quartzite, gneiss and amphibolite, and occasional granite boulders in the surface drift, it is probable, however, that for a considerable distance to the west of Kazaure the floor is formed of the sedimentary gneisses with the usual granitic intrusions. The hills around Kazaure are composed of hard blue quartzite much pierced by later veins of white quartz, and striking N.N.E.–S.S.W. with a dip of 30° to the west. This belt can be traced southward to the neighbourhood of Kasuan Kuka, where the quartzites become more micaceous and where some rocky exposures of granular amethystine quartz probably represent the intrusive quartz veins of Kazaure. From Kasuan Kuka to Kano, granites and an occasional quartz vein are the only rocks exposed, while the deep drift effectually prevents the discovery of the relationship between the surrounding rocks and the decomposed diorites of the two flat-topped hills within the walls of Kano. A few miles east of Kano the crystalline rocks disappear under the sands and clays of Katagum and Bornu. There is every reason, however, to believe that they are continuous to the north- east with the granites, gneisses and quartzites of Machina, Zinder and Mounio.

The two detached hills, Dala and Kogon Dutsi, the landmarks of Kano, with their precipitous slopes and tabular summits, 150 feet above the level of the plain, present from a distance a decidedly sedimentary appearance. Clapperton,[21] indeed, described them as consisting of “argillaceous ironstone mixed with pebbles and a rather soft kind of marl,” while Dala, the eastern mount, “is of clay-ironstone and conglomerate lying upon a bed of soft light clay apparently mixed with vegetable remains.” Closer investigation has shown, however, that both hills are of crystalline origin, Dala being composed entirely of earthy decomposition products with a gritty ferruginous cap, while Kogon Dutsi still retains cores or kernels of undecomposed diorite in the midst of the friable earthy material which forms the greater part of the hill. The felspar and hornblende of the original diorite have decomposed in the lower part of Dala into a whitish clay streaked and spotted with dark green chloritic material. In the upper part the colouration is more uniformly white or yellow, the bulk of the iron having been leached out to form the red ferruginous cap. That the latter is itself composite, however, is evident from the occurrence of gritty grains, subangular fragments of quartz and rolled pebbles of ironstone in the vacuolar and concretionary crust. Kogon Dutsi presents a similar composition, with the exception that the material becomes coarser and more granular in the neighbourhood of the kernels of undecomposed rock. The extensive decomposition of the crystalline rocks to which the earthy hills of Dala and Kogon Dutsi bear witness will be found in the sequel (p. 201) to be an important factor in the evolution of the characteristic scenery of the high plains of Hausaland.

PLATE VII

Around Kontagora and beyond the northern limit of the sandstones which occupy the southern portion of the province of the same name, there are found hard biotite gneisses in places well striated with pink felspathic and dark biotitic bands and crossed by numerous pegmatite veins. To the east the crystalline floor is deeply covered with drift, and outcrops are comparatively rare. In the Beri river, however, there is a good exposure of soft felspathic biotite gneisses, with knots and bands of pegmatite, while at Beri a pink granite forms a compact group of rocky hills. The granite possesses a distinctly foliated margin, and is followed to the east by hard hornblende and biotite gneisses much pierced by granite veins, which may be traced to within six miles of Bobi before they are replaced by the softer micaceous series. On account of the extent of the superficial accumulations, however, these softer gneisses are well exposed for the first time only in the rocky bed of the Kara river which forms the boundary between the provinces of Kontagora and Zaria.

Though the structure of the north-eastern part of Kontagora province is complicated in detail, it may be described in general terms as a succession of gneiss and quartzite bands alternating with one another. They strike a few degrees east of north, and probably pass to the east of the Bukwium-Anka phyllites. The quartzites are frequently tourmaline- bearing, and one band west of Kotonkoro carries scales of graphite. This fact is interesting in connection with the graphitic schists reported from a neighbouring part of Zaria.[22] The gneisses are usually biotite gneisses, sometimes markedly banded, at other times indistinctly so. Schistose rocks are seen in places, quartz schists being prominent to the south of Mahorro, where talc schists are also seen. The gneiss and quartzite country is traversed by pegmatite and quartz veins, but intrusions of unfoliated granite are rather uncommon. Most of the turtle-backs are foliated throughout, differing in this respect from those of many other parts of the Protectorate. Where the Sokoto road leaves the province, the boundary is marked by a high sharp ridge of amphibolite. (A. L.)

A tract of broken hilly country, almost entirely carved out of the softer gneissic series, occupies the south-western corner of Zaria province, and a diagonal traverse across the hills from Moriga on the Kara river by way of Garun Gabbas and Zungeru to Kato and Mikonkeli presents a particularly instructive section. The hills which begin about ten miles south-east of Moriga consist between Inga and Garun Gabbas of a succession of quartz schists, quartz muscovite schists, micaceous and hornblendic schists, felspathic gneisses and amphibolites, pierced by numerous quartz reefs and veins and by a few dykes of basalt and porphyry, the whole series possessing an average strike of N. 10° E. Between Garun Gabbas and the Kaduna river ridges of quartz and quartzite rise conspicuously at intervals out of the pebbly drift which covers the surface. In the railway cutting on the right bank of the Kaduna near Zungeru, quartzites and quartz schists are associated with bands of a hard and minutely foliated granulitic gneiss containing knots and lenticles of quartzo-felspathic and chloritic material. In the river bed below the railway bridge the most conspicuous rock is a well-bedded white quartzite striking N. 30° E. and dipping at an angle of 64° to the south-east (Plate VIII, Fig. 2). The quartzite varies much from a massive variety with knots of pyrites and scattered flakes of white mica through a streaky lustrous micaceous quartzite with linear foliation planes, into a well-foliated quartz muscovite biotite schist. The different varieties are often arranged in alternating parallel bands and the whole series is pierced by occasional narrow veins of quartz. The trench which at the Kaduna bridge forms the low water channel of the river has been carved out of a band of hornblende schist intercalated along the strike of the quartzites. The schist, which in places is composed almost entirely of hornblende, but in others interfoliated and streaked with felspar, has been more easily worn away than the quartzite, and grooved and pot-holed by the action of the river. The quartzites and quartz schists, with intercalated sills of hornblende schist, continue eastward to the adjoining channel of the Dago, where they are replaced by finely striated biotite gneisses. At Zungeru micaceous and felspathic gneisses are mingled with soft, thinly bedded and repeatedly folded hornblende mica epidote gneisses or schists. The whole series is pierced in the usual manner by veins of quartz, pegmatite and granite and strikes in a general way N. 10° E. with a dip of 60° to the east. Immediately east of the railway station, however, these rocks give place to well-foliated granulitic gneisses with knots of felspar, striking N. 20° E. and dipping at 45° E. These gneisses, which probably represent more completely altered portions of the sedimentary series, are in places well striated, with the bands wavy and contorted and crossed irregularly by veins of quartz. They are followed in the vicinity of Kwarra-town by coarsely schistose grits which are among the least altered rocks within the Protectorate. Between Zungeru and Gidan Motu there are numerous exposures of green hornblendic beds similar to those at Zungeru. They become, however, more fissile and contorted towards the east. The strike oscillates between N.N.E. and N.N.W., and the dip varies between vertical and high angles to the east or west, while at Gidan Motu there is an intercalated belt of platy biotite gneisses which strike N. 60° W. and dip at 60° S.S.W. Between Gidan Motu and Kato green slaty schists and phyllites are extensively developed and steeply folded at Kato into sharp anticlines and synclines which possess an average strike of N. 20° W. The phyllites enclose much quartz in lenticles and veins and contain abundant knots and spots of pyrites. Beyond Kato the phyllites are succeeded by soft mica schists, which in turn give place to micaceous and felspathic gneisses. The latter around Mikonkeli, Minna and Paiko are replaced by hard composite and in places banded rocks and invaded by extensive masses of granite which usually show a distinct marginal foliation.

Southward the Zungeru belt of quartzites and phyllites disappears under the sandstones of Nupe, but any correlation with the quartzites and schists of Kabba cannot yet be attempted. Northward the belt has been followed as far as Birnin Gwari and from its pronounced development in the neighbourhood of that place, it is probable that it extends considerably farther north again. For twenty or thirty miles north of Tegina there are a number of granite hills strung together on a roughly north and south line, the granite having apparently been intruded along the strike of the schistose rocks. The granites become foliated locally. Between Bugai and Koriga there is a twenty mile belt of phyllites and subordinate mica schists which runs northward to Birnin Gwari, where the phyllites form a number of sharp north and south ridges and are excellently exposed, both in the sides of the ridges and in the intervening streams. The phyllites are traversed by many quartz veins and gold is widely disseminated in the streams. At Koriga there is an extensive development of hornblende gneiss. To the east are mica schists, and graphitic schists are reported from the neighbourhood.[23] The rocks which rise above the plain in the vicinity of Zaria are all of granite. (A. L.)

So far as known, the Zungeru belt is the last prominent group of phyllites which can be traced within the Protectorate. To the east their place is taken by muscovite and biotite schists, which are to be regarded as more completely reconstructed representatives of the original shales, the metamorphic action having apparently been somewhat more intense in eastern and southern Zaria and Nassarawa. Several belts of quartzites and schists are enclosed in the usual manner within a complex of softer and harder gneisses to the east of Minna and Mikonkeli. At Kuta, for example, there is a belt of quartz muscovite schist passing on the one hand into knotted micaceous schists and on the other into quartzites and carrying much sillimanite and magnetite and large lenticles and knots of quartz and tourmaline quartz. The whole series strikes approximately N. 30° E. and dips at 45° N.W. Again between Chiri and Guni there is much soft micaceous schist rising into low rounded and well-wooded hills, striking generally N.–S., well frilled in places, with a high content in ilmenite and numerous knots and veins of tourmaline quartz. Farther east the hills which run from the neighbourhood of Dorka to Kaderri and Kaserimi are composed very largely of quartzites, quartz schists, quartz muscovite schists and micaceous gneisses, striking N. 10° W., dipping at an angle of 60° W., and pierced by quartz veins, muscovite pegmatites and granites. Between these more prominent belts occur numerous smaller exposures of schists and quartzites and of both the softer and the harder gneisses. To the east of Kaserimi and Duchi and in the valley of the Kaduna the surface is so deeply and extensively covered with drift that little rock beyond the marginally foliated biotite granites of the isolated hills is available for investigation. There are, however, indications from occasional fragments in the drift and rare exposures in the stream courses that a considerable extent of the buried surface is composed of a softer gneissic series. To the east of the Kaduna, the Ruruma and Kerko hills which rise so prominently from the south-eastern plains are composed throughout of a coarse-grained biotite granite, locally carrying hornblende in addition and in places distinctly foliated on the margin. The junction with the gneisses is only rarely seen on account of the thick masses of drift which envelop the base of the hills. At Guri on the eastern slope of the Ruruma hills there are exposures of finely laminated hornblende epidote gneisses, while the granites and the marginal rocks are pierced by numerous dykes of basalt, dolerite and quartz porphyry.

The high plains of south-eastern Zaria are occupied by mica schists and gneisses, with granite hills in one or two localities. Along the eastern border of the province the Rukuba plateau ends in a ragged edge of biotite granite which veins the gneisses at its foot. The country around Jengre and Limoro is interesting on account of the occurrence of masses of soda granite and especially on account of numerous intrusions of dyke rocks, felsites often with flow structure, porphyries and quartz porphyries. Tinstone is frequently met with in the streams of the district. Similar dyke rocks are exposed around the base of the Kudaru hills to the north-west of Leri and there, in addition, dykes and veinlets of basalt penetrate the granite. These hills are made up of hornblende granites with syenitic modifications, and locally, towards the boundary, the granite takes on a distinctly foliated structure, with rounding of the porphyritic felspars as in augen-gneiss. (A. L.)

In Nupe, between the Gurara river and the sandstones of the Niger valley, the crystalline series is introduced by muscovite biotite gneisses, quartzites, hornblende and talc schists and amphibolites, pierced by extensive masses of granite around Gaung, Lefu and Paiko. In the bed of the Gurara, between Gaung and Izon, soft gneisses and hornblende epidote rocks alternate with hard and well-foliated gneisses striking a little west of north, and varying in dip from vertical to 40° or 50° E. Five miles beyond Izon, the Abuja road ascends over a crumpled and contorted series of biotite, hornblende, and epidote hornblende gneisses much injected by masses of biotite granite. Beyond Abuchi a second ascent over soft micaceous gneisses, striking N. 10° W. and vertically foliated, leads up to the striated, ribboned and banded gneisses which have been invaded by the biotite granite of Abuja. The banded gneisses strike N.–S., and are much granitised in the immediate neighbourhood of the intrusive rock, while the granite itself is marginally foliated and crossed by crush lines at the Izon gate (_v._ Traverse No. V, p. 68). Partially foliated granites and banded and composite gneisses continue to within five miles of Kao, whence a belt of soft micaceous gneisses and quartz muscovite schists, striking N. 20° E., stretches eastward to the neighbourhood of Kari. At Kari the escarpment of the northern tableland is formed of soft biotite gneisses, much pierced by veins of quartz and tourmaline quartz. Eastward by way of Kukwi, Anta, Tatara and Jaginde the escarpment shows much granite, associated both with banded gneisses and with softer micaceous and hornblendic rocks. The gneisses and granites are extensively invaded by muscovite and tourmaline pegmatites, and by irregular intrusions of basalt and dolerite. Between Jaginde and Darroro quartzites, quartz schists and mica schists outcrop along the escarpment, and at Darroro a similar series of thin quartzites and gneisses, striking generally N.–S., is pierced by extensive masses of quartz-muscovite-tourmaline pegmatite, and capped by the basalts of the Morrua plateau. The pegmatite is rudely banded parallel with the strike of the gneisses, the various bands being distinguished by their richness in tourmaline. All proportions may be found between quartz muscovite and quartz tourmaline rock, the tourmaline in the latter being either in large well-formed prisms, or in thin alternating bands of felted fibrous crystals. North- east of Darroro the vertical granite cliffs of the Kagoro massif form the margin of the Bauchi plateau, while in the neighbourhood of Assab the later basalts much obscure the crystalline rocks of the lower plains. At Assab the escarpment is largely composed of biotite granite, in places porphyritic, and apparently intrusive into a series of micaceous gneisses which strike N. 10° E. and dip at an angle of 40° to the west.

The northern tableland and the central plains of Nassarawa are composed very largely of the softer gneisses with scattered outcrops of harder composite rocks. Quartz muscovite schists occur in a broad belt on the tableland in the vicinity of Aribi, and calc-silicate rocks between Kwai and Jaginde. The composite and banded gneisses reach their greatest development in the Karshi hills between Abuja and Keffi. The Mada hills and the greater part of eastern Nassarawa are still unexplored. In southern Nassarawa a broad belt of quartz muscovite schists, mica schists, hornblende schists and chlorite schists extends between Gwombe and Buga, the mica schists in the neighbourhood of Zagabutu being particularly rich in staurolite, garnet and imperfectly crystallised knots of andalusite, presumably of contact origin. The whole series has been steeply folded on a meridional axis and then coarsely corrugated transversely, the result being that individual exposures may strike at any angle between E.–W. and N.–S. The average strike is inclined around Gwombe to the north-west, and around Buga to the north-east. To the east of Buga the schists give place to the usual varied succession of softer gneisses, pierced in the usual manner by veins of quartz and tourmaline quartz, and associated from time to time with banded and composite rocks and detached masses of granite. Numerous dykes of basalt and quartz porphyry appear to be associated with the granites of the Kurudu and Anagoda hills, which lie respectively west and south-east of the town of Nassarawa.

The crystalline rocks of Bauchi province present similar characters to those of other parts of the Protectorate. The broad belts of schists and quartzites found in other provinces are here, however, conspicuously absent, such types being represented only by thin bands intercalated within the softer gneisses. Banded and composite rocks, on the other hand, appear to be present in somewhat greater proportion than elsewhere, while domes and turtlebacks of foliated or partially foliated granite (Plate XII, Figs. 1, 2) are in places particularly abundant. The softer and harder rocks are, however, so intimately mingled that a detailed description of the succession is impossible. Excellent exposures of the harder series are to be found in the Kogin Delimi below Naraguta in the west of the province, and of the softer gneisses in the Jarawa hills to the south of Bauchi, and of both in the broken rocky country between Kanna and Angass in the southern portion of the province (_v._ Traverse No. VI, p. 68). The strike of the gneisses, while very variable, appears to have a greater tendency to assume an E.–W. direction in Bauchi than in other provinces, a tendency which is probably due to the greater intensity of the transverse forces which have elsewhere corrugated the prevailing meridional strike of the gneisses and schists. This feature is particularly marked in the neighbourhood of the Ningi massif, which is elongated approximately along the strike of the surrounding softer and harder gneisses.

Another notable feature of the province of Bauchi is the number and variety of the later igneous intrusions. Pegmatites, quartz reefs and veins of tourmaline quartz are as abundant in Bauchi as elsewhere, but the attention is somewhat diverted from these commoner intrusive types by the presence of an abundant series of dyke rocks, and of extensive masses of non-foliated granite and syenite. The pegmatites are in places particularly rich in ilmenite, rutile and chalcedonic quartz; the dyke rocks present almost every variety between basalt and quartz porphyry; and many of the granites are especially rich in soda, and in places show chilled margins at their junction with the gneisses. The summit of the plateau around Bukuru is occupied by one of these extensive granitic intrusions, which is of peculiar interest from the fact that in places it is veined and impregnated with tinstone and other minerals. A similar granite, also carrying tinstone, forms the greater part of the Kwandokaya hills to the north of Toro. Many intrusions of syenite are found around and to the east of the town of Bauchi. The Ningi massif is largely composed of a soda granite, with a well-chilled margin, accompanied by numerous dykes of granite and porphyry, and in places carrying tinstone. Similar granites are found at Kila and Fagam on the borders of Bauchi and Kano, at Shira in Katagum to the north, and at Gurkawa on the Muri border to the south. The Murchison Hills are believed to be largely granitic, but they and the southern margin of the plateau are still practically unexplored. A small detached area of granite and gneiss, pierced by numerous pipes of basalt and trachyte, is found around Tangalto and Chongwom in the south-east of Bauchi province.

_Southern Muri and Yola._ (A. L.)

The strip of crystalline rocks which runs along the southern boundary of Muri and Yola has an average width of forty miles and is separated from the central area by the sedimentary rocks of the Benue plain. The crystalline floor appears at the surface, however, in a number of small inliers around Arofu in western Muri, and thus quite near to the river. Five miles east of Arofu a small area of fresh biotite-granite about fifty yards in diameter peeps from beneath the limestone. Immediately north of the town itself there is a knoll of granitoid gneisses striking north and south. On proceeding southward along this line of strike limestone is passed over for three miles, but then the gneiss appears at the surface again, and this time forms a ridge running southward for at least some miles. It may well be that other inliers of crystalline rocks occur in the unexplored Munchi country.

Farther south the transition from sedimentary to crystalline rocks is not accompanied by any feature in the drift-covered plain. Though the exposures are not good, it appears that biotite-gneisses and schists with the customary northerly strike are penetrated by granite veins. The rocky masses which lie south and south-west of Takum are probably largely or entirely of granite. The bulk of the material which forms the scattered hills around and to the north of Takum is also granite, but here it is inextricably mixed up with gneiss which is met with in patchy fashion from top to bottom of the hills. The foothills of Shebshi are schistose grits and gneisses invaded by granites in the usual manner.[24]

PLATE VIII

Only the northern part of the Mumuye-Manna plateau has been traversed. The rock there is almost always biotite-granite, but it is by no means always of the same variety. Occasional quartz veins are seen on the plateau and also on the lower ground between Numnai and Panteti, where the quartz carries pyrites. Intrusions of quartz-porphyry are often seen cutting through the granites, _e.g._, east of Numnai, around Shega Mundi hill, around Panti Baturi and at Yendam Hill. The granites only occasionally carry included blocks and fragments of gneiss, but on descending to the plain biotite-gneiss is met with striking N. 10° E., and the plain from Chukol eastward to the Vere Hills is occupied by biotite-gneiss, with some intrusions of granite. The Vere Hills themselves are made up almost entirely of granite, though to a small extent of gneiss also. Biotite-gneiss is frequently seen round the base of the plateau, generally much penetrated by granite or quartz-porphyry. Thus, speaking generally, the plains of southern Yola are mainly of gneiss and the plateau areas mainly of granite.

To the north of the Benue plain again, the country around Song, Goila and Korubi is a biotite-granite area with some conspicuous dykes of quartz-porphyry running for miles across the country. Near Zummu porphyry rises just above the soil of the plain and exhibits flow- structure, while between Zummu and Holma a felsite dyke is seen cutting through the granite. The hills around Holma are built of a well-jointed biotite-granite, and the sides of the hills are concealed by the loose blocks split off along the joints. The granite of the Kilba Hills, near Pella, is a very fresh, coarsely-crystalline biotite-granite, and locally, where the Kilba Hills rise out of the plain to the west, the granite takes on an augenstructure. In consequence of exfoliation (Plate VIII, Fig. 1) most of these hills rise up as smooth bare rocks, but some are made up of heaps of blocks, and it is not possible to explain the contrast as due to a difference in material. Between the hills around Holma and those around Pella there is a plain of gneiss about fifteen miles broad. The gneiss has been formed by the injection of a micaceous gneiss or schist with innumerable strings and veins of granite. The exposures are excellent throughout, but the gneiss never forms hills.

The country north of Pella is comparatively uninteresting. It consists of an extensive plain with isolated hills and rocks of biotite-granite. The nature of the rocks underlying the plain is practically unknown. The small area of the Kwaba Hills is composed of biotite-granite with some quartz-porphyry. (A. L.)

Crystalline rocks, largely of a gneissic character, are exposed also in southern Bornu around Kwoia to the east of the Gongola and along the base of the escarpment on the north-western margin of the Burra volcanic plateau. In eastern Bornu the granite rocks of Chibuk and Kobshi are the first prominent outcrops of crystalline rocks to the south of Maidugari. Barth,[25] however, mentions an isolated granitic cone, Mt. Deladeba, to the N.N.E. of Isge, while Courtet,[26] Gentil and Freydenberg[27] note the occurrence of knobs of crystalline rock between Lake Chad and Lake Fittri. It is highly probable indeed that the thick alluvial accumulations to the east and south-east of Chad and in the lower Shari valley rest for the most part upon a crystalline floor, provided with scattered relics of Tertiary sandstone. (J. D. F.)

[Footnote 18: Hubert, _Miss. Sc. au Dahomey_, 1908, p. 325.]

[Footnote 19: Parkinson, _Q. J. G. S._ (63), 1907, p. 317.]

[Footnote 20: Barth, _Central Africa_, 1857, Vol. II, pp. 41, 42.]

[Footnote 21: Denham and Clapperton’s _Narrative_, 1826, pp. 50, 57.]

[Footnote 22: Dunstan, _Colonial Reports—Miscellaneous_, No. 26, p. 15.]

[Footnote 23: Dunstan, _Colonial Reports—Miscellaneous_, No. 26, p. 15.]

[Footnote 24: See also Downes, _Yola and Cross River Boundary Delimitation_, 1910, p. 35.]

[Footnote 25: Barth, _Central Africa_, 1857, Vol. II, p. 369.]

[Footnote 26: Courtet, _C. Rd. Ac. Sc._, 140, p. 160.]

[Footnote 27: Gentil and Freydenberg, _Bull. Soc. Geol. Fr._ (4), VIII, 1908, p. 44.]