Chapter VII: Introduction: 92

Under the term "metamorphism," considered philologically, any change may be included that is undergone by rocks after their original deposition. Van Hise, in his monumental treatise, covers processes of cementation and alteration by percolating waters, as well as those larger changes that accompany earth-movement and the transference of rocks into regions of igneous activity. It is, indeed, impossible to draw any just line in this matter; but there is a general agreement that "metamorphic rocks" are those that have been altered by heat or pressure or both, either on a local or a regional scale, with the result that new structures, or new minerals, or both, have arisen in the mass. The efficacy of heat alone or of pressure alone, of _contact-metamorphism_ or of _dynamo-metamorphism_, in producing considerable changes has been much debated. Some of the thermal changes have been already referred to in the chapter on igneous rocks. While, moreover, the new structures and the development of mica in ordinary slate bring it into the metamorphic group, we have found it convenient to describe the slates in connexion with common clays. The rocks now to be dealt with give evidence of more extreme changes, and the crystalline character of their constituents is appreciable by the unaided eye. For the most part, then, this chapter treats of _gneisses_ and _schists_. The wider use of the terms _schiste_ and _schiefer_ on the continent of Europe makes it necessary in most countries to style the metamorphic forms "crystalline schists."

Over wide areas of certain countries, and sometimes when we approach the localised cores of mountain-chains, the rocks show a parallel arrangement of their constituents, reminding us of sediments; but their constituents are all crystalline, and they are more interlocked with one another than is the case in ordinary strata.

Such rocks have long been said to be "foliated." The term was used by G. P. Scrope as far back as 1825; but this author, in common with most geologists of his day, regarded the mineral folia as resulting from sedimentation. D'Aubuisson de Voisins[93] had already referred the parallelism of the _feuillets_ of mica in schists to some cause acting on them during the consolidation of the rock from a plastic state; but it was left for Charles Darwin[94], in his remarkable observations on metamorphic rocks in 1846, to separate clearly _foliation_ from stratification.

In all cases of metamorphism, we have to bear in mind that the alteration may be both chemical and physical. Substances may have been removed from the rock, others may have been imported. The crystalline constituents that are now present do not necessarily result from the crystallisation of the original materials of the rock.

MICA AND HORNBLENDE SCHISTS

_Schists_ are the ordinary foliated rocks of fine or medium grain. The folia are really flattened lenticular mineral aggregates, often bent and waved, lying on and against one another, with their platy surfaces in parallel planes. They result (i) from the deformation under pressure of objects already present in the rock, such as pebbles or crystals; or (ii) from the development of minerals under pressure during the process of metamorphism, such minerals being allowed greater facilities for growth in directions perpendicular to that from which the pressure is exerted; or (iii) from the development of minerals, notably mica, along the planes of weakness provided by stratification or by cleavage.

The trend of foliation-planes across a country is often, as Darwin pointed out, remarkably regular; in some cases, it follows that of the stratification, in others that of cleavage. The wrinkling of the foliation must be ascribed to subsequent compression, and all the features seen in the "strain-slip" structure of slate (p. 92) are repeated on a somewhat coarser scale in schists.

Some schists are undoubtedly produced by the contact-metamorphism of shales. On the flanks of mountain-chains, where argillaceous rocks have been arched into domes, and where granite has intruded as a core, the complete passage can be traced from sediment to schist. The clay-rocks lend themselves readily to the production of mica, usually of the pale type. Andalusite, and occasionally sillimanite and kyanite, arise. Andalusite often forms grey prisms of irregular outline, resembling slate-pencils, and standing out above the mica on any weathered surface. Almandine garnet is almost always present. Quartz occurs in streaks and patches, which resolve themselves into granular aggregates on microscopic examination. The mica imparts a distinct foliation to the mass; but the original stratification is very often preserved, and the minerals have developed along its planes. Small differences in the constitution of the original strata give rise to different types of schist, interbedded with one another. Andalusite, for instance, may occur only in certain argillaceous layers, while other layers are quartzose, through the presence of original sand. _Mica-schist_ is the commonest type of metamorphic rock.

Where mineralisation has taken place over a wide area, it may be difficult to say if the foliation-planes in a schist are those of bedding, or of superinduced cleavage, or whether they indicate a sliding movement in the mass under pressure, whereby all preceding structures have become obliterated.

_Amphibole-schist_, often styled _epidiorite_, consists of foliated hornblende, or its greener ally actinolite, associated with granular felspar and sometimes with equally granular quartz. The amphibole being usually prismatic, the crystals are found with their longer axes arranged in parallel planes, and often streaked out parallel to one another. Minute wrinklings, due to subsequent yielding, are not so frequent as in mica-schists. Amphibole-schists occur commonly as knots and somewhat irregular masses among mica-schists, and represent basic igneous rocks that were interbedded or intrusive in the sedimentary series. The pyroxene of the original rock has become recrystallised as hornblende, and the felspathic constituent has rearranged itself in granular forms. J. J. H. Teall[95] has described in interesting detail an example from the older rocks of Sutherland, and his paper contains a useful discussion of problems of pressure-metamorphism.

AMPHIBOLITES

Hornblende-schists are often seen to pass into true diorites; but they also have relationships with the more puzzling rocks known as _amphibolites_. These, again, graduate into _pyroxenites_, or rocks rich in pyroxene, with granular quartz and triclinic felspar, and into _eclogites_, which may be defined as pyroxenites with garnet.

Pyroxene-eclogite, in South Africa, is associated with diamond[96], and fragments of exploded eclogite abound in the igneous vents from which the diamonds are extracted.

What has been called "pyroxene-granulite" is a dark granular eclogite, including rhombic pyroxene side by side with garnet, and associated, in Saxony and Skye, with igneous intrusions. In both localities it has been shown to result from the inclusion of basic rocks, such as dolerites and gabbros, in a bath of some invading magma. The lens-like form of the Saxon masses, and the occurrence also of sheets of pyroxene-granulite interlaminated with fine-grained granite, were till lately attributed to the rolling-out action of pressure-metamorphism. By what H. Credner calls a complete reversal of opinion, due mainly to the opening of new railway-sections, the granular eclogites of Saxony are now regarded as products of extreme contact-alteration, combined with igneous flow[97]. A. Harker[98] similarly points out that examples in Skye are derived from basaltic lavas, into which gabbro has intruded, producing a complete reconstruction of the rock.

Where a series of igneous rocks and sediments, in some cases already altered by pressure, has been attacked and partly melted up by granite, amphibolite-blocks are found as the common residue in the mingled mass. The quartzites and mica-schists of the mantle that overlies the granite dome may have disappeared by stoping and absorption (see p. 126). Rocks rich in amphibole remain, and they commonly contain pyroxene as well as hornblende. In some cases, as in Skye and Saxony, they may be traced to basic igneous rocks; but in others they may be referred with equal certainty to limestone. The interaction of the granite magma and the calcareous sediment has produced a silicate rock completely different from either.

Lévy[99] and Lacroix have shown how the amphibolites of France may sometimes represent dolerites, sometimes limestones. Their work has recently received striking support from the observations of the Geological Survey of Canada[100]. Streaky hornblende-gneisses over wide areas of Ontario are now attributed to the partial absorption of overlying limestone by what was once regarded as a "fundamental" granite. The amphibolite blocks have become drawn out into bands that follow all the flow-structure of the invading igneous mass. A small area of the same kind was studied in 1900 in north-west Ireland[101], where a remarkably pure granitoid rock, consisting of quartz and alkali felspar, has become enriched with dark mica at the expense of blocks of amphibolite included in it.

METAMORPHIC MARBLES AND QUARTZITES

Some of the changes that convert limestone into crystalline marble have already been referred to on pp. 36 and 54. The presence of mica in limestones may allow of foliation when pressure comes to be applied to them, and _calc-schists_ result. The mica may be detrital, or may arise through the metamorphism of clayey bands; but it forms weak layers, along which the shearing movements take place which lead to a schistose structure in the mass. Pure granular marble may also occasionally become converted into a calc-schist, by deformation of its crystalline grains along gliding planes within each crystal.

When we consider quartzites, the same question rises as in the case of crystalline limestones, and it is often difficult to state that a quartzite owes its characters to metamorphism. Microscopic examination sometimes reveals the effects of earth-pressures in the crushed and powdered condition of the larger grains; and no rocks exhibit the power of such pressures in producing structural modifications more strikingly than the coarse quartz-grits that are sometimes involved in regions of dynamic metamorphism. Pebbles and grains are alike deformed, pressed out along planes of fracture, and finally reduced to bands of powdered quartz. When felspathic pebbles occur in these grits, the resulting schistose mass has almost the appearance of a banded igneous rock, and streaky white mica may arise from the alteration of potassium felspar.

Some sandstones contain sufficient felspar or calcium carbonate to form a flux when they are subjected to thermal metamorphism. At times a glass thus arises between the grains, and reacts upon the original quartz. When the igneous magma has melted up a sandstone or a quartzite, blocks of the sediment may remain surrounded by a mixed and recrystallised product from both rocks. Wright and Bailey[102] have studied an example in Colonsay, where a hornblende rock has partly dissolved a quartzite, the residual blocks being surrounded by "halos" of interaction, composed of quartz and alkali felspar.

GNEISSES

_Gneisses_ may be broadly defined as banded crystalline rocks in which felspar is visible to the unaided eye. Though this will include many igneous masses, it is doubtful if a more rigid description can be given. Numerous gneisses, in fact, owe their parallel structures to flow while in a molten state. Others are rocks that have been deformed by pressure, and their constituents have become drawn out along planes of solid flow. Where actual shearing has taken place, the minerals in the close neighbourhood of the planes of movement may become especially modified, ground down, and deformed. The foliated structure may then be marked by the appearance of differentiated bands. Such bands may also arise from the spreading out under pressure of certain large constituents, such as porphyritic crystals of felspar, which produce white bands, or of pyroxene, which will become modified into granular amphibole and will produce dark streaks through the rock.

Gneisses may also result from the intrusion of felspathic igneous rocks, in sheets of varying thickness, between the layers of a sediment or a schist (Fig. 19); or from the intrusion of one igneous rock into another, with varying degrees of interaction and absorption.

It has often been presumed that the invaded igneous rock must have been in such cases in a plastic state. The supply of heat within the earth during such processes, and the action of the gases, corroding, as Doelter says, "like a blowpipe-flame," are, however, clearly sufficient to melt down large blocks, the residue being then carried forward as wisps or bands in the invader.

Many strikingly banded gneisses are thus of composite origin. Their felspathic granitoid bands can be traced in the field to an igneous source, while their darker and usually micaceous layers can as surely be attributed to the invasion and incorporation of adjacent schists (Fig. 20). But it is quite possible that in other cases the banded gneiss is a sedimentary rock which has undergone what Judd[103] has styled "statical metamorphism." The differences in successive bands are then due to original differences in successive strata; one has yielded a granitic layer, one a layer of quartzite, one, which was more argillaceous, a layer of mica-schist. The bands in such a gneiss record the stratification.

Gneisses are often described as if they consisted of layers of various minerals, quartz, felspar, and mica, alternating one with another. As a matter of fact, a gneiss may exist in which there is no differentiation into layers; the whole of the constituents have been drawn out and elongated, any mica present becoming naturally conspicuous by its flattened wisp-like forms. The banded gneisses, on the other hand, where layer-structure is obvious, consist in reality of bands of different rock-types. Sometimes all the layers are granitoid, but one band will contain only quartz and felspar, while another will contain the same minerals with an admixture, and perhaps a great predominance, of mica.

G. P. Scrope[104] made an immense step forward when he realised in 1825 that such banded rocks, "the inferior crystalline zones," might be pushed out of position and "protruded" among others "in a solid or nearly solid state." He goes on, "The protrusion of the foliated rocks, gneiss, mica-schist, clay-slate, etc. was chiefly occasioned by their peculiar structure; the parallel plane surfaces of their component crystals, particularly the plates of mica, sliding with facility over one another; while the laminar structure of these rocks was in turn increased during this process, the crystals being elongated in the direction of their motion, as in the case of the clinkstones and pearl-stones of the trachytic formation." After this, there was little left for the later advocates of dynamo metamorphism to put forward.

While Darwin[105] recognised how the granite at Cape Town had worked its way insidiously between the layers of a schist, it was left for Michel Lévy to emphasise the part played by what is called _lit-par-lit_ injection in the making of banded gneiss (see p. 120). K. A. Lossen, Johann Lehmann, and other distinguished workers in Germany made clear, on the other hand, the effects of pressure in moulding and reforming crystalline rocks, and even in bringing about the crystallisation of certain minerals in a previously sedimentary mass.

The dynamo-metamorphic school assumed immense importance from 1884 onwards, the date of the publication of Lehmann's work on "Die Entstehung der altkrystallinischen Schiefergesteine," and for a time the intrusion of igneous masses was held, both in Germany and the British Isles, to have had a merely local significance as a metamorphic agent. Wherever "regional metamorphism" was spoken of, pressure-effects were held to be predominant. Indeed, the profound modifications that may occur in rocks when lowered into subterranean cauldrons is only now becoming generally realised. The tendency to regard the structures of large masses of gneiss as of necessity due to deformation and shearing in a solid state has, however, passed away[106].

Pressure-effects are of course clearly traceable in most gneisses, and are of immense importance in many metamorphic areas; but we find again and again that gneissic structure has been injured rather than developed by crushing subsequent to the consolidation of the rock. In some cases, where this structure is due to igneous flow, which of course often took place under considerable pressure, even the puckerings of the stratified or foliated rock which was invaded by the igneous magma have been followed by the invading sheets. In other cases, as in the composite amphibolite gneiss of Canada, or the similar rocks of the Ox Mountains in Ireland, the contortions in the mingled mass are clearly due to the viscid flow of the consolidating invader.

The growing appreciation of the views on recurrent thermal metamorphism that were originally propounded by James Hutton in 1785 has led to the assignment of far younger ages to many masses previously regarded as "fundamental" and Archæan. Some of these rocks are undoubtedly of high antiquity, but are found to be intrusive in strata of a late pre-Cambrian series. Others, such as the material of the Saxon laccolite, and the gneisses on the north-east Bohemian border, are now known to be of Upper Palæozoic age.

THE QUESTION OF A FUNDAMENTAL GNEISS

Ever since A. C. Lawson[107] showed in Canada how the Laurentian gneiss had invaded and swallowed up the overlying Huronian rocks, suspicion began to fall on the doctrine of a "fundamental" gneiss. We may now well ask ourselves the following questions:--

(i) Was there a time in the early history of our globe when schists and gneisses were deposited as a prevalent type of sediment, under conditions which have not since recurred?

(ii) If so, which of the characters of these pre-Cambrian rocks are original, and which have been acquired through subsequent metamorphism?

(iii) On the other hand, is the prevalence of gneiss and schist in early pre-Cambrian groups of rock due to the fact that, the older the rock, the more metamorphism, by recurrent heat and pressure, it is likely to have undergone?

(iv) We may prefer the theory of Laplace, that the earth is cooling from a molten state; or the planetesimal theory, according to which heat has been developed during the consolidation and contraction of an agglomerate of solid particles; yet in either case we must admit that the earth's outer layers were once nearer to the heated parts of the earth than they are now. Is it not likely, then, that early sediments became frequently immersed in baths of molten matter, and that contact-metamorphism and admixture on a regional scale have produced in them the characters that have been attributed to a fundamental gneiss[108]?

J. J. Sederholm[109] has traced in Finland four groups of Archæan sedimentary material, which have been successively invaded by granite from the depths. The bare wave-swept isles of Spikarna, east of Hangö, serve as models of structures that are traceable throughout the Baltic lands. The more we regard the oldest gneisses of one region after another, the more we see in them igneous matter that has attempted to assimilate sediments of still older date. The banded structures that have been appealed to as indicating the power of earth-movements to deform the solid crystalline crust prove, in very many cases, to record the foliation of rocks that were already metamorphosed before the igneous matter spread among them. In some of these cases, this foliation followed planes of original stratification, and we are forced to conclude that true sedimentary structure may after all control the features of a gnarled and contorted fundamental gneiss. We are still far from discovering the primitive crust formed about a molten globe, and the brilliant proofs of evolution in the organic world are unmatched by any evidence of the evolution of rock-types during geological time.

METAMORPHIC ROCKS AND SCENERY

Metamorphic rocks are usually associated with the scenery of mountain, moor, and forest. The highly altered siliceous masses furnish but indifferent soils. The connexion between metamorphic rocks and earth-crumpling, and their frequent penetration by granite, lead to the production of rugged ridges and high moorlands, among which denudation has cut romantic glens. The schists weather out on the valley-walls along their foliation-surfaces, and scarps arise like those of stratified rocks. The face of such a scarp is broken away in a zigzag and splintery fashion, and the sharp edges of the foliated mass stand out like teeth upon the sky-line. Gneisses associated with the schists present a contrast of smoother surfaces, wherever denudation has been long continued. Foliated diorites and amphibolites, however, may produce wild crags that even overhang; while recently exposed gneiss, at high altitudes, may give rise to pinnacles and serrated forms.

Where alternations of quartzite and mica-schist occur, irregularities of the surface are readily maintained. Heather climbs upon the yellow soils furnished by the schist, and trees may gather in its hollows; but the quartzite stands out bare and dominant. In some cases the upturned beds of the latter weather out like dykes across the country.

Worn-down plateaus of ancient gneiss, the mere residues of mountain-land, may be seen in the storm-swept levels of the Outer Hebrides, and in the hummocky country, a swelling sea of bare grey rock and peat-filled hollows, that borders all the west of Sutherland. The irregular weathering of mica-schist, and the readiness with which it can be carved by streams, control the bold landscapes of the highlands from the Trossachs to Lough Ness, and thence away again to the northern sea. Here and there, great domes of intrusive granite rise amid the broken moorlands; at times, a white cone of quartzite catches the eye with a gleam like that of snow. We may traverse this country as an introduction to the high glacial plateaus and deeply notched seaward slopes of the metamorphic lands of Norway; or to the contrasts of jagged schists and resisting gneisses that meets us as we near the Alpine core.

REFERENCES

(_The numbers of volumes are given throughout in thick type; the
dates are between brackets, and the page-references follow in
ordinary figures._)

[Footnote 1: Cordier, "Mémoire sur les substances dites en masse,
qui entrent dans la composition des Roches Volcaniques," Journ.
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[Footnote 2: On specific gravity of mineral grains see especially
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[Footnote 3: Sorby, Q. Journ. Geol. Soc. London, =14= (1858), 453.]

[Footnote 4: Katzer, "Geologischer Führer durch Bosnien," IX
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[Footnote 5: A. W. Rogers, "Geology of Cape Colony," ed. 2 (1909),
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[Footnote 6: Linck, "Die Bildung der Oolithe u. Rogensteine," Neues
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[Footnote 7: Daly, "The Limeless Ocean," Amer. Journ. Sci., Ser. 4,
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[Footnote 8: A. R. Horwood, Geol. Mag. (1910), 173; and Cole and
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[Footnote 9: "The Atoll of Funafuti," Roy. Soc. London (1904).]

[Footnote 10: M. Ogilvie (Gordon), "Coral in the Dolomites," Geol.
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[Footnote 11: Gardiner and Reynolds, "The Portraine Inlier (Co.
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[Footnote 911: Walther, "Einleitung in die Geologie als historische
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[Footnote 12: See Nichols, Field Columbian Museum, Geology, =3=
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[Footnote 13: Skeats, "Limestones from upraised coral islands,"
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[Footnote 14: See generally W. Meigen, "Neuere Arbeiten über die
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[Footnote 15: Skeats, "Origin of the Dolomites of southern Tyrol,"
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[Footnote 16: Pfaff, "Beiträge über die Entstehung des Magnesits u.
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[Footnote 17: Garwood, "On the origin of the concretions in the
Magnesian Limestone of Durham," Geol. Mag. (1891), 433.]

[Footnote 18: Skeats, _op. cit._, ref. 15, p. 135.]

[Footnote 19: J. J. H. Teall, "On dedolomitisation," Geol. Mag.
(1891). 513, and Rep. Brit. Assoc. (1903).]

[Footnote 20: J. S. Howe, "Geology of Building Stones" (1910), 353.]

[Footnote 21: Hinde, "On Beds of Sponge remains in the south of
England," Phil. Trans. (1885), Pt. 2, 427.]

[Footnote 22: Sollas, "On the structure of the genus Catagma," Ann.
and Mag. Nat. Hist., Ser. 5, =2= (1878), 361. Also _ibid._, 6
(1880), 447.]

[Footnote 23: Cayeux, "Étude micrographique des Terrains
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[Footnote 24: Jukes-Browne, "The amount of disseminated silica in
the Chalk in relation to flints," Geol. Mag. (1893), 545.]

[Footnote 25: Guppy, "Observations of a Naturalist in the Pacific:
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[Footnote 26: Rogers, _op. cit._, ref. 5, p. 403.]

[Footnote 27: Judd, "On the unmaking of Flints," Proc. Geol.
Assoc., =10= (1887), 217. Also Hintze, "Handbuch der
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[Footnote 28: Grund, in Stille's "Geologische Charakterbilder,"
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[Footnote 29: Rullmann, "Handbuch der technischen Mykologie," =3=
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[Footnote 30: Hinde, "Catalogue of Fossil Sponges," Brit. Mus.
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[Footnote 31: Rogers, "Geology of Cape Colony," ed. 1 (1905), 373.]

[Footnote 32: _Ibid._, 357.]

[Footnote 33: Lyons, "Libyan Desert," Q. Journ. Geol. Soc., =50=
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[Footnote 34: Victorian Naturalist, =27= (1910), 90.]

[Footnote 35: Sorby, "Structure and origin of non-calcareous
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[Footnote 36: Phillips, "Constitution and history of Grits and
Sandstones," _ibid._, =37= (1881), 6.]

[Footnote 37: A. Daubrée, "Geologie expérimentale" (1879), 256.]

[Footnote 38: Phillips, _op. cit._, ref. 36, p. 26.]

[Footnote 938: J. Barrell shows how wind-borne sand may form a
covering to the dry and sun-cracked surface of a lake-deposit;
"Relation between climate and terrestrial deposits," Journ.
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[Footnote 39: Lake and Rastall, "Text-book of Geology" (1910), 297.
Compare C. Lapworth, "Intermediate Text-book of Geology" (1899),
176, and "Geological Structure of N. W. Highlands," Geol. Surv.
Scotland (1907).]

[Footnote 939: See A. B. Searle, "The Natural History of Clay"
(1912).]

[Footnote 40: Hall, "The Soil," ed. 2 (1908), 34, and E. J.
Russell, "Clay," Standard Cyclopedia of Modern Agriculture
(1908).]

[Footnote 41: Reade and Holland, "Sands and Sediments," Proc. Liv.
Geol. Soc. (1903-6).]

[Footnote 42: Andrussow, "La Mer Noire," Guide des Excursions,
vii^{me} Congrès géol. internat. (1897).]

[Footnote 43: B. Smith, "Upper Keuper Sandstone," Geol. Mag.
(1910), 302. Compare F. Cresswell, Trans. Leicester Lit. and
Phil. Soc. (1910).]

[Footnote 44: J. Murray and A. Renard, "Deep Sea Deposits,"
Challenger Rep. (1891), 231.]

[Footnote 45: _Ibid._, 234.]

[Footnote 46: _Ibid._, 229.]

[Footnote 47: Harker, "Slaty Cleavage and allied rock-structures,"
Rep. Brit. Assoc. (1885).]

[Footnote 48: Leith, "Rock Cleavage," Bull. U. S. Geol. Surv., No.
239 (1905).]

[Footnote 49: Lamplugh, "Geology of Isle of Man," Mem. Geol. Surv.
Gt. Brit. (1903), 72-86.]

[Footnote 50: Darwin, "Geological Observations on S. America"
(1846), chap. vi.]

[Footnote 51: Reade and Holland, "Green Slates of the Lake
District, with a Theory of Slaty Cleavage," Proc. Liv. Geol.
Soc. (1900-1), 124.]

[Footnote 52: A. Harker, "On 'eyes' of Pyrites &c.," Geol. Mag.
(1889), 396.]

[Footnote 53: T. N. Dale illustrates an extreme case, "Slate
Deposits of U.S.," Bull. U.S. Geol. Surv., No. 275 (1906), 31.]

[Footnote 54: Harker, _op. cit._, ref. 47, p. 19.]

[Footnote 55: Leith, _op. cit._, ref. 48, p. 152.]

[Footnote 56: I. Russell, "Glaciers of N. America" (1897), 25.]

[Footnote 57: See, for instance, T. W. Edgeworth David, "Evidences
of glacial action in Australia," Q. Journ. Geol. Soc., =52=
(1896), 289.]

[Footnote 58: For general discussions of Igneous Rocks, see J.
J. H. Teall, "British Petrography" (1888); H. Rosenbusch,
"Mikroskopische Physiographie," ed. 4 (1905-7); F. Zirkel,
"Lehrbuch der Petrographie," ed. 2 (1894); A. Harker, "Natural
History of Igneous Rocks" (1909); J. P. Iddings, "Igneous
Rocks," 1 (1909).]

[Footnote 59: Cross, Iddings, Pirsson, and Washington,
"Quantitative Classification of Igneous Rocks" (1903).]

[Footnote 60: Harker, _op. cit._, ref. 58, p. 186.]

[Footnote 61: Iddings, _op. cit._, ref. 58, p. 130 &c.]

[Footnote 62: _Ibid._, pp. 228-241.]

[Footnote 63: Scrope, "Considerations on Volcanos" (1825), 141.]

[Footnote 64: G. A. J. Cole and J. W. Gregory, "Variolitic Rocks of
Mt Genèvre," Q. Journ. Geol. Soc., =46= (1890), 311.]

[Footnote 65: A. Geikie, "Ancient Volcanoes of Gt Britain,"
=1= (1897), 25. Also C. Reid and H. Dewey, "Pillow lava of
Cornwall," Q. Journ. Geol. Soc., =64= (1908), 264.]

[Footnote 66: Anderson, "Volcano of Matavanu," _ibid._, =66=
(1910), 632.]

[Footnote 67: Dewey and Flett, "British Pillow lavas," Geol. Mag.
(1911), 202 and 241.]

[Footnote 68: Steinmann, "Die Schardtsche Ueberfaltungstheorie
&c.," Ber. nat. Gesell. Freiburg i. B., =16= (1905), 44.]

[Footnote 69: Doelter, "Petrogenesis" (1906), 33 and 109-123.]

[Footnote 70: Harker, _op. cit._, ref. 58, p. 82.]

[Footnote 71: Iddings, _op. cit._, ref. 58, p. 280.]

[Footnote 72: Brögger, "Die Eruptionsfolge bei Predazzo,"
Vidensskab. Skrifter (1895), No. 7, p. 152.]

[Footnote 73: Daly, "Secondary origin of certain Granites," Am.
Journ. Sci., Ser. 4, =20= (1905), 185, with useful references to
Bayley and others.]

[Footnote 74: Hawes, "The Albany granite and its contact
phenomena," _ibid._, Ser. 3, =21= (1881), 31.]

[Footnote 75: G. A. J. Cole, "Geology of Slieve Gallion," Sci.
Trans. R. Dublin Soc., =6= (1897), 242.]

[Footnote 76: Daly, "Mechanism of igneous intrusion," Am. Journ.
Sci., Ser. 4, =15= (1903), 269, and later.]

[Footnote 77: For a recent review in favour of this theory, see
Loewinson Lessing, "The fundamental problems of Petrogenesis,"
Geol. Mag. (1911), 248 and 289.]

[Footnote 78: Darwin, "Geological Observations on volcanic islands"
(1844), chap. VI.]

[Footnote 79: Brögger, "Die Eruptivgesteine des Kristianiagebietes"
(1894 &c.).]

[Footnote 80: Harker, _op. cit._, ref. 58, chaps. XIII and XIV.]

[Footnote 81: Daly, "Origin of the alkaline rocks," Bull. Geol.
Soc. Am., =21= (1910), 108, and "Magmatic differentiation in
Hawaii," Journ. Geol., =19= (1911), 309. See, however, H. I.
Jensen, as to primitive accumulation of alkalies in the upper
layers; "The distribution of Alkaline Rocks," Proc. Linn. Soc.
N. S. W., =33= (1908), 521.]

[Footnote 82: Schwarz, "Causal Geology" (1910).]

[Footnote 83: Doelter, _op. cit._, ref. 69, pp. 71-213.]

[Footnote 84: Judd, "On Tertiary gabbros &c.," Q. Journ. Geol.
Soc., =42= (1886), 54.]

[Footnote 85: Harker, _op. cit._, ref. 58, p. 90, and Nature (Sept
1911), 319. See also Jensen, ref. 81, p. 522.]

[Footnote 86: Dewey and Flett, _op. cit._, ref. 67, p. 245.]

[Footnote 87: Steinmann, _op. cit._, ref. 68, p. 64.]

[Footnote 88: See especially W. J. Sollas, "The volcanic district
of Carlingford," Trans. R. I. Acad., =30= (1894), 502.]

[Footnote 89: A. Geikie, _op. cit._, ref. 65, =2=, 344 and fig.
348.]

[Footnote 90: Branner, "Decomposition of rocks in Brazil," Bull.
Geol. Soc. Am., =7= (1896), 255.]

[Footnote 90: Macculloch, "Description of the Western Islands of
Scotland," =1= (1819), 267.]

[Footnote 92: For general discussions of Metamorphic Rocks,
see A. Delesse, "Études sur le Métamorphisme des Roches"
(1858); Lehmann, "Untersuchungen über die Entstehung der
altkrystallinischen Schiefergesteine" (1884); A. Geikie,
"Text-book of Geology" (1903), 764-807 and 728; Van Hise, "A
Treatise on Metamorphism," U. S. Geol. Survey, Mon. 47 (1904);
U. Grubenmann, "Die krystallinen Schiefer," ed. 2 (1909); A.
Geikie and others, "The Geological Structure of the N. W.
Highlands of Scotland," Mem. Geol. Surv. Scotland (1907).]

[Footnote 93: D'Aubuisson de Voisins, "Traité de Geognosie" (1819),
=1=, 298.]

[Footnote 94: Darwin, ref. 50.]

[Footnote 95: Teall, "Metamorphosis of Dolerite into
Hornblende-Schist," Q. Journ. Geol. Soc., =41= (1885), 133.]

[Footnote 96: T. G. Bonney, "The parent rock of the diamond in S.
Africa," Geol. Mag. (1899), 309.]

[Footnote 97: R. Lepsius, "Geologie von Deutschland," 2ter. Teil
(1903), 146 and 169; H. Credner, "Die Genesis des sächsischen
Granulitgebirges," Renuntiations-programm (1906).]

[Footnote 98: Harker, "Igneous Rocks of Skye," Mem. Geol. Surv.
Scotland (1904), 115.]

[Footnote 99: Lévy, "Excursion à Aydat," Bull. Soc. géol. France
(1883), 916; "Granite de Flamanville," Bull. Carte géol. France
=5= (1893), 337.]

[Footnote 100: F. D. Adams, "Haliburton and Bancroft areas," Mem.
Geol. Surv. Canada, No. 6 (1910), 120.]

[Footnote 101: G. A. J. Cole, "Metamorphic rocks in E. Tyrone and
S. Donegal," Trans. R. I. Acad., =31= (1900), 453.]

[Footnote 102: W. B. Wright and E. B. Bailey, "Geology of
Colonsay," Mem. Geol. Surv. Scotland (1911), 28.]

[Footnote 103: Judd, "Statical and dynamical metamorphism," Geol.
Mag. (1889), 246.]

[Footnote 104: Scrope, _op. cit._, ref. 63, p. 234.]

[Footnote 105: Darwin, _op. cit._, ref. 78, chap. VII.]

[Footnote 106: See especially J. Horne and E. Greenly, "Foliated
Granites &c. in E. Sutherland," Q. Journ. Geol. Soc., =52=
(1896), 633.]

[Footnote 107: Lawson, "Geology of Rainy Lake Region," Ann. Rep.
Geol. Surv. Canada for 1887 (1888).]

[Footnote 108: Compare Chamberlin and Salisbury, "College Text-book
of Geology" (1909), 428, and other works by these authors.]

[Footnote 109: Sederholm, "Om granit och gneis i Fennoskandia"
(with English summary), Bull. Comm. géol. Finlande, No. 23
(1907), and elsewhere.]

TABLE OF STRATIGRAPHICAL SYSTEMS

Quaternary Group

Post-Pliocene and Recent

Cainozoic Group

Pliocene
Miocene
Oligocene
Eocene

Mesozoic Group

Cretaceous
Jurassic
Triassic

Palæozoic Group

Permian
Carboniferous
Devonian
Gotlandian (= Silurian or Upper Silurian)
Ordovician (or Lower Silurian)
Cambrian

Pre-Cambrian Group

INDEX

(_"Ref" indicates that the name is quoted in the list of references, pp. 162-169._)

Acid igneous rocks, 127, 132
Adams, F. D., 125, ref. 100
Africa, S., 148.
See Cape of Good Hope and Rhodesia.
Agassiz, A., 25;
L., 98
_Agents minéralisateurs_, 107
Algæ, calcareous, 25
Alkaline igneous rocks, 129
Alps, 14, 16, 23, 138, 143, 162
Ammonites, 23
Amphibole-Schist, 147
Amphibolite, 148
Anderson, T., 117
Andesite, 133
Andrussow, N., 84
Antrim, Co., 46, 135
Aragonite, deposition of, 17;
in shells, 22, 86
Armitage, 64
Ash, 88, 111
Assimilation in igneous rocks, 128
Atlantic and Pacific types of igneous rocks, 130
Auvergne, 112
Axmouth, 46

Bacteria, extraction of iron by, 61
Bagshot Heath, 73
Bailey, E. B., ref. 102
Banded structure, 120
Barrell, J., ref. 938
Barrois, C., 125
Barytes in sandstone, 62
Basalt, 132, 135
Basic igneous rocks, 127, 132
Batholites, 123
Bavaria, dolomites of, 32
Belemnites, 23
Black Sea, 17, 84
Bohemia, 134, 158
Bonney, T. G., ref. 96
Boulder-clay, 96
Bournes, 43
Brachiopods, 24
Branner, J. C., 140
Brazil, 88, 140
Breccia, 55
Brögger, W. C., 125, 128
Brongniart, A., 2
Bunsen, R. W., 127

Cader Idris, 143
Calcareous Tufa, 14, 16
Canada, 103, 150, 158
Cañons of Arizona, 47
Cape of Good Hope, 16, 41, 59, 63, 103, 121, 136, 156
Causses, 45, 48, 50
Cayeux, L., 39
Cephalopods, 23
Chalk, 20, 42
Chamberlin, T. C., 129
Chara-limestone, 19
Cheddar, 48
Chert, 40, 62
China-clay, 86
Christiania district, 125, 128
Christmas Island, 37
Clare, Co., 46
Clay, 78
Cleavage, 89
Close, Maxwell H., 98
Cole, G. A. J., 117, refs. 8, 75 and 101
Coleman, A. C., 103
Colonsay, 152
Columnar structure, 115
Composite gneiss, 122, 153
Cones, volcanic, 112, 133
Conglomerates, 70
Connemara marble, 36
Contact metamorphism, 144
Conybeare, W. D., 35
Coral-reefs, 25;
silicification in, 40
Cordier, P. L. A., 3
Cork marble, 54
Credner, H., 125, 149
Crinoidal limestone, 24
Cross, W., ref. 59
Crush-conglomerates, 28
Crystallisation in igneous rocks, 107

Dale, T. N., ref. 53
Daly, R. A., 18, 33, 125, 127, 128
Dana, J. D., 30
Darwin, C., 25, 90, 128, 145, 156
Daubrée, A., 66
D'Aubuisson de Voisins, 145
David, T. W. E., ref. 57
Dedolomitisation, 35
De la Beche, H., 18
Delesse, A., ref. 92
Derbyshire, 48, 73, 97
Desert sands, 68, 71
Dewey, H., 117, 130
Diatoms, 40
Differentiation in igneous rocks, 128
Dinaric Alps, 16, 23, 52
Diorite, 132
Doelter, C., 18, 31, 125, 130, 154
Dolerite, 132
Dolinas, 50
Dolomite, 12, 26, 29, 30
Donegal, Co., 137, 150, 153
Down, Co., 74, 137
Dreikanter, 71
Drumlins, 98, 102
Durham, dolomite of, 35
Durocher, J., 127
Dwyka Conglomerate, 103
Dykes, 110, 118, 137
Dynamo-metamorphism, 144

Eclogite, 148
Edinburgh, 143
Egypt, 22, 64, 68
Ehrenberg, C. G., 5, 20
Epidiorite, 147
Eurite, 132
Eutectic proportion, 109
Exfoliation of granite, 140

Felsitic structure, 108
Ferromagnesian minerals, 109
Fiji Is., 40
Fingal's Cave, 116
Finland, 159
Flagstones, 69
Flett, J. S., 117, 130
Flint, 38, 62;
gravels, 74
Flocculation of clay, 80
Flow-cleavage, 92
Fluidal structure, 120
Foliation, 90, 145
Foraminifera, 20
Forehammer, G., 29
Fracture-cleavage, 92
Freshwater molluscs, 23
Fuji-yama, 134
Funafuti atoll, 19, 26
Fundamental gneiss, 158
Fusulina limestone, 21

Gabbro, 132, 142
Gardiner, C., 29
Garwood, E. J., 35
Geikie, A., 117, 138, 142
Giant's Causeway, 116
Gilbert, G. K., 123
Glacial gravels, 98
Glaciers, arctic, 98
Glassy igneous rocks, 110
Glauconite in chalk, 20
Globigerina-ooze, 20
Gneiss, 122, 152, 158, 161
Gordon, M. Ogilvie, 27
Granite, 132, 138
Granodiorite, 132
Great Salt Lake, Utah, 15
Great Whin Sill, 136
Greenly, E., ref. 106
Gregory, J. W., 117
Greywacke, 58
Grund, A., 50
Guppy, H. B., 40

Halimeda, 19, 29
Hall, A. D., 81
Harker, A., 89, 107, 125, 128, 130, 149
Harlech Beds, 74
Hawaii, 106
Hawes, G. V., 126
Hebrides, 116, 135, 152, 161
Hegau, the, 135
Henry Mountains, Utah, 123
Hercegovina, karstland, 14, 52
Highlands of Scotland, 76, 143, 161
Hinde, G. J., 38, 62
Holland, P., 83, 90
Hornblende-Schist, 147
Horne, J., ref. 106
Horwood, A. B., ref. 8
Howe, J. A., 13
Hutton, J., 41, 104, 122, 158
Hydrozoa, 25

Iddings, J. P., 108, 115, 125, ref. 59
Igneous Rocks, 103
India, 140
Intermediate igneous rocks, 127, 132
Intrusion of igneous rocks, 124
Intrusive sheets, 122, 136
Irish Channel, limestone in, 17
Iron-bacteria, 61
Iron Pyrites in muds, 85

Jajce, 16
Jensen, H. T., ref. 81
Judd, J. W., 6, 42, 68, 104, 130, 154
Jukes-Browne, A., 40
Jura Mts., 46

Kalahari desert, 41, 63
Kaolin, 87
Karlsbad, 14
Karst, 49
Katzer, F., 16
Kerry, 76
Klement, C., 31
Knoll structure, 28

Laccolites, 123
Lacroix, A., 15, 125, 150
Lake, P., 76
Lamellibranchs, 22
Lamplugh, G. W., 89
Landslips, 46, 94
Lapworth, C., ref. 39
Laterisation, 64
Laurentian gneiss, 158
Lautaret Pass, 95
Lava-flows, 113
Lava-plains, 114
Lawson, A. D., 125, 158
Lehmann, J., 157
Leinster granite, 143
Leith, C. K., 89
Leith Hill, 73
Leonhard, K. von, 3
Lepsius, R., 125, ref. 97
Lessing, L., ref. 77
Lévy, M., 6, 125, 150, 156
Limestones, 12, 150;
deposited from solution, 14;
organic, 19
Linck, G., 16, 18, 61
_Lit-par-lit_ injection, 157
Lithoidal structure, 108
Lithothamnium, 20, 29
Little, O. H., ref. 8
Llanberis, 96
Loam, 82
Londonderry, Co., 135
Lossen, K. A., 157
Lower Greensand, 62, 73
Lundy Id., 139
Lyons, H. G., 63

Macculloch, J., 142
Magmas, igneous, 127
Magmatic differentiation, 128
Magnesian limestone, 35
Magnesium in organic skeletons, 29
Marble, 36, 54, 150
Marl, 83
Martel, E. A., 52
Matopo Hills, 140
Matterhorn, 143
Metamorphic Rocks, 143
Mica-Schist, 147, 161
Millepora, 25
Millersdale, 48
Minerals, 6, 8
Mojsisovics, E., 27
Monaghan, Co., 74
Mont Blanc, 138, 143
Mont Genèvre, 117
Mull, 135
Murray, J., 25

Nagelfluh, 14
New Forest, 74
Northumberland, 136
Norway, 162
Nubian Sandstone, 63
Nummulitic limestone, 21

Obsidian, 132
Old Red Sandstone, 75
Oolitic grains, 15, 17
Oolitic Limestone, 18, 40
Ophicalcite, 36
Order of crystallisation of minerals, 108
Ox Mountains, 158

Paris basin, 40, 74
Petrographical provinces, 130
Pfaff, 30, 34
Phillips, J. A., 64, 67
Phillips, W., 35
Phosphatic limestone, 36
Phosphorites du Quercy, 37
Pillow-structure, 117
Pipe-clay, 78
Pisolite, 15, 18
Planetesimal theory, 129, 130, 159
Plutonic conditions, 119
Porosity of sandstone, 66;
of clay, 79
Porphyritic structure, 119
Portland stone, 18
Portrane, ref. 11
Purbeck Marble, 54
Pyroxenite, 148

Quartz veins, 56, 65
Quartz-felsite, 132
Quartzite, 63, 76, 151, 161
Quartz-porphyry, 132

Radiolaria, 40, 118
Ravines in limestone, 48
Reade, T. M., 83, 90
Red Clay of deep seas, 88
Regional metamorphism, 157
Reynolds, S. H., 29
Rhodesia, 140
Rhyolite, 132
Richthofen, F. von, 25, 27
Ripple-marks, 69
Rock, definition of, 7
Roestone, 15
Rogers, A. W., 41, 62, 63
Rosenbusch, H., 6, ref. 58
Rothpletz, A., 27
Russell, E., 82
Russell, I., ref. 56

Samoa, 117
Sand-dunes, 62, 69
Sand-rock, 65
Sands, origin, 56;
cementing of, 60;
grains, 66
Sandstones, 56;
"crystalline," 64
Saxony, 148, 149, 158
Sea, action of on shore, 58, 87;
calcium carbonate in, 16
Searle, A. B., ref. 939
Sederholm, J. J., 125, 159
Semper, K., 25
Serpentine, 133
Schists, 145, 161
Schwarz, E. H. L., 129
Scoriæ, 112
Scoriaceous structure, 106
Scrope, G. P., 104, 116, 145, 156
Shale, 83, 96;
colours of, 85
Sharpe, D., 89
Shell-marl, 23
Silicates in igneous rocks, 109
Silicified wood, 64
Sills, igneous, 136
Skeats, E. W., 30, 31, 35
Skye, 135, 138, 142, 149
Slate, 88, 96
Smith, B., 86
Snowdon, 143
Sollas, W. J., 38, refs. 2 and 88
Sorby, H. C., 5, 64, 66, 89, 90
Southern Uplands, 74
Spherulites, 108
Spilitic lavas, 117, 131
Spitsbergen, 20, 81, 99, 101
Sponges, siliceous, 38, 62
Steinmann, G., 118, 131
Stoping process, 126
Strain-slip cleavage, 92
Sun-cracks, 69
Surrey Hills, 43, 73
Swallow-holes, 44
Sweden, gneiss of, 155
Syenite, 132

Teall, J. J. H., 117, 148
Terra rossa, 50
Terrace-structure in limestone, 46;
in basalt, 135
Thames, material in solution, 17
Torridon Sandstone, 76
Tors, 138
Trachyte, 133
Travertine, 15
Tridacna, 23
Trieste, 50
Tuff, 111
Tyrol, dolomites, 26, 31, 53

Ultrabasic igneous rocks, 132

Van Hise, C. R., 143
Vesuvius, 111
Victoria, Australia, 64
Volcanic ash, 88, 111;
cones, 112, 133;
dust, 111;
necks, 122, 134;
tuff, 111

Walther, J., 29
Weald, 73
Weathering in tropics, 64, 140
West Indies, 18, 37
Whinstone, 137
Wright, W. B., 152

Yellowstone Park, 15
Yoredale, 73

Zirkel, F. von, 6, ref. 58

=Cambridge:=
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Transcriber Note

Minor typos corrected. The volume used endnotes which had several duplicated numbers with a _bis_ extension. Those numbers were changed to 900 + the original number. There is no anchor for Footnote 37 and 6 and 7 have more than one anchor in the text.