Chapter XXV: Appendix: III

NOTES ON THE GEOLOGY OF HASSANEIN BEY’S EXPEDITION, SOLLUM-DARFUR,
1923

F. W. Moon

HAVING been requested by Hassanein Bey, in the absence of Dr. Hume, to report upon the interesting geological specimens collected by him on his recent exploratory tour through the Western Desert from Sollum, on the southern shores of the Mediterranean Sea, to Darfur in the Sudan, I have much pleasure in submitting the following short summary of the main features of the journey from a geological point of view, as deduced from the specimens collected.

Although the specimens are small and, in the case of the igneous rocks, much weathered, yet for all intents and purposes they are quite sufficient to enable one to make deductions and conclusions as to the main geological formations passed over.

As the explorer explains, there was not the freedom of transport he would have desired for making a larger collection of full-sized specimens, nor did he wish to incur the displeasure of those who formed his escort by seeming to do anything that might appear in any way suspicious, such as the constant breaking and collecting of stones.

From the tabulated list of specimens herewith appended, it may be seen that the beginning of the journey was made over Miocene formations indicated by fossil oysters and pectens (identified as _Ostrea digitalina_, _O. Virleti_, _Chlamys zitteli_, and others), of which there are seven valves of the former, two of the second, two of the third, and five which resemble _Chlamys sub-malvinæ_.

The Miocene formations continue through Siwa, Jaghbub, and Jalo, and far southward to a point about 180 km. to the south of Jalo (see Specimens 1-4), where the last Miocene specimen (No. 4) was picked up.

At this point begins a wide barren stretch of flat and desolate country, devoid of rocks of any particular geological interest beyond the thin covering of “sand and alluvium” which extends for nearly 200 km. (four days of monotonous marching) along the route to the south.

Then, when about 50 km. north of Zieghen, the explorer noticed that the aspect of the country took a sudden change; the light yellow of the Miocene limestones and desert sands gave place to brighter, more varied, and highly colored scenery, which the specimens show was due to the approach of the Lower Cretaceous Nubian Sandstones, which, wherever they occur, add beauty to the landscape in unbelievable brilliancy and variety of coloring.

Although greens and blues are sometimes observed, red is the dominant color, all shades of pink, terra-cotta, maroon, and brick red are blended together, and umbers and ochers are sometimes present.

Thus we are provided with a point well westward of any hitherto known on the northern limits of the Nubian Sandstone; in other words, the evidence suggests the continuation westward by some 600 km. of the Nubian boundary shown on the existing 1:1,000,000 colored geological map of Egypt (1910).

Mention may here be made of the apparent absence of specimens definitely representing the younger Cretaceous rocks which are shown colored green, on the map just referred to, to the east; but this may be readily accounted for by the presence of the expansive plain passed over between Jalo and Zieghen, the “sand and alluvium” covering of which is quite sufficient to hide all evidence of the younger rocks of that age.

Another question rendered obscure by the presence of this large flat expanse of “sand and alluvium” is the exact position of the southern limits of Miocene formations. If we assume (which seems to be the case) that the point, 180 km. to the south of Jalo, where the last Miocene specimens were collected, be the southern boundary of the Miocene, then we find that the distribution of that formation as now suggested is of particular interest as (1) indicating a western extension of the general outline of the ancient Mediterranean Ocean in Miocene time; and (2) adding extra weight to our conceptions as regard the period (in geological chronology) of the Egyptian-Sinai uplift which caused the elevation of land involving a considerable portion of Egypt in pre-Miocene times, and ultimately defining the shores of the Miocene Sea more or less as we now follow it from this newly discovered spot between El Harrash (Zieghen) and Jalo to a short distance east of Siwa from where it runs northeastward to the thirtieth degree of latitude, along which it continues with little variation to Suez.

It would appear that all Egypt lying between the Miocene Gulf of Suez in the east and the Siwa-Zieghen Miocene shore-line in the west and southward was dry land in Miocene times, and therefore exposed to denudation extending over a vast geological period during which the Nubian Sandstone and younger Cretaceous rocks were laid bare and in a position to have Miocene strata laid down upon or against them.

The Nubian Sandstone, as indicated by the Specimens 5-10, shows identical characteristics to those exhibited wherever it occurs in Egypt or Sinai. It is a sandstone consisting in the main of more or less fine rounded grains of pure quartz, through which are distributed, in greater or lesser proportions, larger grains or pebbles. In cases where the latter predominate, a pudding-stone or conglomerate results; where the larger grains are sparsely distributed, a porphyritic appearance is suggested.

The cementing material, which may be either calcareous, silicious, or ferruginous, is the color-imparting medium, the depth of color depending upon the amount and composition of iron oxides present in it, and when this material is weathered, washed out, and accumulated into pockets it becomes, when finely ground, suitably adapted for the manufacture of paints.

The Nubian Sandstone continues south of the Miocene-Nubian boundary above referred to, to a point some 15 km. north of the Arkenu Mountains.

Approaching this point, still another change in the general aspect of the country met the eye of the explorer: the brighter coloring of the sandstones gave place to the duller browns, grays, and blacks of an abrupt range of igneous hills, the position on the ground where the igneous rocks come up through the Nubian Sandstones being approximately 25 km. north of Arkenu.

The changes of scenery in passing from one formation to another are strikingly seen in the excellent collection of photographs procured by Hassanein Bey, who is to be highly complimented on the success he obtained under great difficulties and inconveniences.

Judging from the Specimens 11-12 submitted for examination, the igneous rocks, of which the Arkenu-Ouenat Hills are composed, consist mainly of coarsely crystalline granites and syenites varying in texture and appearance, and through them run intrusive veins of finer-grained dike rocks.

The Arkenu Hills are mostly composed of rocks represented by Specimens 12 and 14, which are really somewhat similar in composition.

No. 12 consists of a holocrystalline aggregate of large crystals of a light gray (possibly a decomposed or kaolinized orthoclase) alkali-felspar, which constitutes the main bulk of the rock. No quartz is visible in the hand specimen, which is greatly weathered, and only gives a specific gravity of a little over 2.5. Small crystals of dark greenish hornblende are well formed, and occur in fewer numbers than in Specimens 17 and 21, which are representatives of the rockmass of the Ouenat Hills to be described presently.

Specimen 14 is an unweathered gray rock chiefly composed of a mottled gray alkali-felspar, with hornblende crystals in similar numbers to those in Specimen 12.

The microscopic examination of a rock-section made from Specimen 14 corroborates the above description, but introduces the possibility of the presence of nephelin in granular-like patches in the slide, which correspond to darker slightly lustrous areas in the hard specimen; however, no nephelin has been actually identified.

Specimens Nos. 12 and 14 may therefore be called syenites.

Running through the syenites of the Arkenu Hills are veins of intrusive rocks represented by Specimens Nos. 11, 13, and 15, and no doubt many others occur.

Specimen No. 11 represents a vein of a hemicrystalline, hard dark green rock weathering brown on the outer surface, with innumerable small dark specks which are scarcely discernible in the unweathered portion of the specimen.

Under the microscope this rock is found to be of considerable interest. It consists of an aggregate of small phenocrysts of idiomorphic felspar, which in places assumes the appearance of a crypto- or microcrystalline felspathic matrix crowded with acicular crystals of a green mineral resembling ægirine. The latter are in places irregularly distributed, but in areas where the felspar occurs in roughly rectangular or lozenge form, the ægirine microlites are crowded round the edges of the latter.

No quartz is noticeable in the rock-slide, and the rock may be tentatively determined as an ægirine-felsite, apparently similar to a rock described and figured in Harker’s “Petrology for Students.”

A second vein in the Arkenu Hills is represented by Specimen No. 13, which is a brownish quartzite.

The third vein is indicated by Specimen No. 15, which is a dark gray laminated rock, weathering to reddish brown, very fine in texture, with small clear crystals of phenocrysts sparsely disseminated through the ground-mass. The rock-slide shows, under the microscope, a great similarity to Specimen No. 11 as described above. The felspar here, however, forms a much finer crypto-crystalline ground-mass than in the former, and the ægirine microlites are smaller, tapering, and not so well formed. This rock may also be tentatively called a fine ægirine-felsite.

The Ouenat Hills are mainly composed of rocks represented by Specimens 17 and 21, the chief constituent of which is a gray alkali-felspar (possibly orthoclase with some microcline). Quartz is well represented in idiomorphic forms; no mica is noticeable in the hand specimens, but many well-developed prismoidal crystals of very dark or dark green hornblende are thickly distributed throughout the mass.

No slide was made of these specimens owing to their fragile condition on account of weathering, but the rock may be determined as a coarsely crystalline gray hornblende granite.

Specimen No. 18 is another representative rock from, and constitutes a considerable bulk of, the Ouenat Hills. It may be termed a red granite, approximating to an aplite with very little mica, which decomposes and forms oxides of iron which have stained the rock a brownish red; quartz and felspar form the main bulk of the rock.

As in the case of the Arkenu syenites, so here in the Ouenat granites we find other examples of endogenous veins running through the parent rock, represented by Specimens 16, 19, and 22.

Specimen No. 16 represents a vein of purplish felsite, in the felsitic ground-mass of which occur phenocrysts of idiomorphic felspar.

Specimen No. 19 represents a vein of pure white granular quartz rock which occurs in and may have been the cause (through denudation) of the cave found in the foot-hills of the Ouenat range.

Specimen No. 22, found at Garet Shezzu, is a typical quartzite which may also occur as a vein in the granites.

Two specimens found inside the cave in the Ouenat Oasis are of particular interest. These specimens are Nos. 20 and 21. The former, a laminated travertine, could only have been deposited from running water, as the formation of ripple-markings confirms; and from notes made by the explorer at the time of his inspection, we learn that there was quite a lot of it lying about on the floor of the cave. Under the microscope spheroidal structure is displayed, representing the ripple-markings, and in the matrix of calcite many fragments of quartz, felspar, etc., are conspicuous, these having been derived from the denuding granites. No organic remains were observed.

The second specimen (No. 21) is a fragment of the hornblendic granite of which the Ouenat Hills are chiefly composed, and which forms the roof of the cave; this is coated on one side with a thin black iron-manganese film, similar to the well-known deposit on the rocks in the Nile at or near the Asswan Dam.

This whole igneous area, which includes the newly discovered mountains and oases of Arkenu and Ouenat, is possibly of limited extent, and occurs in, and is possibly surrounded by, a much wider expanse of Nubian Sandstone, in a similar manner to several other such areas already shown on the 1:1,000,000 geological map of Egypt.

Judging from other known similar occurrences where igneous rocks appear surrounded by Nubian Sandstones, we may infer that the latter were deposited upon the ancient igneous rocks which subsequently rose vertically, bending the superincumbent strata over and round them, possibly only to a slight degree in this instance, as none of the photographs show very highly dipping rocks.

On leaving Ouenat for Erdi the igneous area is soon left behind. The actual junction between it and the Nubian Sandstone (which again forms the surface rock) occurs at a point about 20 km. south of Ouenat, and the aspect of the country again changes from the more uneven weathering of the igneous range of grays and browns to brilliant colorings of the Nubian Sandstone which forms a long series of prominent escarpments rising to heights bordering on 1000 meters above the sea between Enebah and Kuttum, after which the ground begins to fall, until El Fasher is reached, where the ground-level is only about 700 or 800 meters in height.

=Summary.=—The several interesting geological features suggested by the recent expedition may be summarized as follows:

1. That Miocene rocks extend southward, to or near the twenty-seventh degree of north latitude, forming a large bay bordered by older rocks.

2. That the Miocene rocks, resting here apparently upon or against Nubian Sandstones, seem to follow the same conditions noticed first by Dr. Hume in the Gulf of Suez region, namely, that they rest upon older and older formations the further southward they are found; in other words, that prior to the deposition of the Miocene there was a time of denudation which was more effectual in the south than in the north, owing to the fact that in the south was an area of greater uplift.

3. That a large area of Nubian Sandstone (Cretaceous) exists south of this latitude.

4. That a newly discovered range of igneous rocks in the Arkenu Hills exists well inside Egyptian territory, and possibly entirely within this Nubian Sandstone area, or connected as a tongue to a larger igneous massif lying in the south.

5. That the Cretaceous rocks (younger than the Nubian Sandstone) which appear on the colored Geological Map of Egypt further to the northeast, possibly do not occur along the route traversed, having been concealed beneath “sand and alluvium” areas.

LIST OF SPECIMENS COLLECTED BY AHMED MOHAMMED HASSANEIN BEY IN HIS
EXPEDITION FROM SOLLUM TO DARFUR

_No._ _Date_ _Locality (As taken from _Determination of
1923 descriptions on labels)_ Specimens_

1 — Siwa Three crystal fragments
of selenite; one pecten
and two oyster shells
(possibly of Miocene age)

2 — Jaghbub One pecten in a shelly
limestone (possibly
indicating Miocene age)

3 — Surface rocks on the way One piece of fossilized
from Jaghbub to Jalo wood; three loose
silicious pebbles; two
long concretions of
calcareous grit, and
fibrous salt crystal
(curved, five inches long)

4 March In Wadai, in small Two loose pebbles of
20 patches calcareous grit with
quartz grains

5 March Getting near El Harrash Nubian Sandstone
24 (Zieghen), patches of (one specimen)
this strewn (broken)
before reaching Halab

6 March One day’s journey from Hard ferruginous bands
28 El Harrash (Zieghen), on from Nubian Sandstone
way to Kufra (five specimens)

7 March Garet El Sherif Nubian Sandstone
29 (three specimens)

8 — Gebel El Neri, _garas_ Ferruginous bands
nearing Hawari (color maroon) from
Nubian Sandstone (three
specimens); one black
ferruginous “bomb”

9 — Kufra (Taj) hills Blocks of Nubian
Sandstone (three
specimens)

10 April Between Kufra and Nubian Sandstone
22 Ouenat; specimens from (one specimen);
chain of hills crossed and ferruginous
that day bands from Nubian
Sandstone (two
specimens)

11 April Arkenu Mountain Igneous rock
24 (ægirine felsite)

12 April From patches of Arkenu “ (badly weathered
24 Mountain; there are big syenite)
hills formed entirely of
it . . . on the outside
of the mountain

13 April From big patches north “ (quartzite vein in)
24 of Arkenu Mountain

14 April From Arkenu Mountain “ (gray syenite)
25 itself

15 April Found in big blocks “ (ægirine felsite)
25 buried in the ground on
the border of Arkenu
Mountain, in the Arkenu
valley

16 — Specimen from formations “ (felsite)
(in layers) found in
Ouenat big valley

17 — Ouenat Mountain is “ (badly weathered
mostly of this stone hornblende-granite

18 — Stone of which Ouenat is “ (badly weathered
generally formed granite)

19 — Found inside the “ (quartz vein)
water-cave at Ouenat,
near the water-level;
plenty of it in small
patches

20 — Found inside the Calcareous deposit from
water-cave at Ouenat running water (travertine)

21 — Found inside the Igneous rock
water-cave at Ouenat, in (badly weathered
the roof; most of the hornblende-granite,
stone of the cave and coated with polished
mountain is of this kind film of iron which may
have come from the water)

22 May 8 From Garet Shezzu, near “ (fine quartzite)
Ouenat

23 May 10 Between Ouenat and Erdi Nubian Sandstone
(one specimen)

24 May 13 Found strewn on red Ferruginous band
sand, getting near Erdi; (containing hematite) from
nothing but red sand and Nubian Sandstone
this stone

25 May 16 Erdi Hills Dark red clay, with small
percentage of quartz sand
(grinds up into dark
brick-red powder)

26 May 16 Stone of Erdi Hills Brick-red clay, with small
percentage of quartz sand
(grinds easily into
bright, brick-red colored
powder)

27 May 19 Agah Hills Soft, fine, yellow to red,
slightly calcareous
micaceous sandstone

Transcriber's note:

pg 220 Added: ".”" after: "though your camp is far away"

pg 231 Changed: "and not the the intrinsic value" to: "and not
the intrinsic value"

pg 236 Changed: "ground near Baret Shezzu" to: "Garet"

caption of illustration facing pg 260, Changed: "WOMAN OF THE
PALLATA TRIBE" to: "FALLATA"