CHAPTER VI. Metamorphic Rocks

METAMORPHIC ROCKS

INTRODUCTION

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 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, 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 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, 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. A. Harker 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 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. 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, 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 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 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 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 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.

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 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?

J. J. Sederholm 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.)

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:= PRINTED BY JOHN CLAY, M.A. AT THE UNIVERSITY PRESS

THE CAMBRIDGE MANUALS OF SCIENCE AND LITERATURE

Published by the Cambridge University Press under the general editorship of P. Giles, Litt.D., Master of Emmanuel College, and A. C. Seward, F.R.S., Professor of Botany in the University of Cambridge.

+------------------------------------------------------+ | A series of handy volumes dealing with a wide | | range of subjects and bringing the results of modern | | research and intellectual activity within the reach | | both of the student and of the ordinary reader. | +------------------------------------------------------+

HISTORY AND ARCHAEOLOGY

42 Ancient Assyria. By Rev. C. H. W. Johns, Litt.D.

51 Ancient Babylonia. By Rev. C. H. W. Johns, Litt.D.

40 A History of Civilization in Palestine. By Prof. R. A. S. Macalister, M.A., F.S.A.

78 The Peoples of India. By J. D. Anderson, M.A.

49 China and the Manchus. By Prof. H. A. Giles, LL.D.

79 The Evolution of New Japan. By Prof. J. H. Longford.

43 The Civilization of Ancient Mexico. By Lewis Spence.

60 The Vikings. By Prof. Allen Mawer, M.A.

24 New Zealand. By the Hon. Sir Robert Stout, K.C.M.G., LL.D., and J. Logan Stout, LL.B. (N.Z.).

85 Military History. By the Hon. J. W. Fortescue.

84 The Royal Navy. By John Leyland.

76 Naval Warfare. By J. R. Thursfield, M.A.

15 The Ground Plan of the English Parish Church. By A. Hamilton Thompson, M.A., F.S.A.

16 The Historical Growth of the English Parish Church. By A. Hamilton Thompson, M.A., F.S.A.

68 English Monasteries. By A. H. Thompson, M.A., F.S.A.

50 Brasses. By J. S. M. Ward, B.A., F.R.Hist.S.

59 Ancient Stained and Painted Glass. By F. S. Eden.

80 A Grammar of English Heraldry. By W. H. St J. Hope, Litt.D.

ECONOMICS

70 Copartnership in Industry. By C. R. Fay, M.A.

6 Cash and Credit. By D. A. Barker.

67 The Theory of Money. By D. A. Barker.

86 Economics and Syndicalism. By Prof. A. W. Kirkaldy.

LITERARY HISTORY

8 The Early Religious Poetry of the Hebrews. By the Rev. E. G. King, D.D.

21 The Early Religious Poetry of Persia. By the Rev. Prof. J. Hope Moulton, D.D., D.Theol. (Berlin).

9 The History of the English Bible. By John Brown, D.D.

12 English Dialects from the Eighth Century to the Present Day. By W. W. Skeat, Litt.D., D.C.L., F.B.A.

22 King Arthur in History and Legend. By Prof. W. Lewis Jones, M.A.

54 The Icelandic Sagas. By W. A. Craigie, LL.D.

23 Greek Tragedy. By J. T. Sheppard, M.A.

33 The Ballad in Literature. By T. F. Henderson.

37 Goethe and the Twentieth Century. By Prof. J. G. Robertson, M.A., Ph.D.

39 The Troubadours. By the Rev. H. J. Chaytor, M.A.

66 Mysticism in English Literature. By Miss C. F. E. Spurgeon.

PHILOSOPHY AND RELIGION

4 The Idea of God in Early Religions. By Dr F. B. Jevons.

57 Comparative Religion. By Dr F. B. Jevons.

69 Plato: Moral and Political Ideals. By Mrs J. Adam.

26 The Moral Life and Moral Worth. By Prof. Sorley, Litt.D.

3 The English Puritans. By John Brown, D.D.

11 An Historical Account of the Rise and Development of Presbyterianism in Scotland. By the Rt Hon. the Lord Balfour of Burleigh, K.T., G.C.M.G.

41 Methodism. By Rev. H. B. Workman, D.Lit.

EDUCATION

38 Life in the Medieval University. By R. S. Rait, M.A.

LAW

13 The Administration of Justice in Criminal Matters (in England and Wales). By G. Glover Alexander, M.A., LL.M.

BIOLOGY

1 The Coming of Evolution. By Prof. J. W. Judd, C.B., F.R.S.

2 Heredity in the Light of Recent Research. By L. Doncaster, Sc.D.

25 Primitive Animals. By Geoffrey Smith, M.A.

73 The Life-story of Insects. By Prof. G. H. Carpenter.

48 The Individual in the Animal Kingdom. By J. S. Huxley, B.A.

27 Life in the Sea. By James Johnstone, B.Sc.

75 Pearls. By Prof. W. J. Dakin.

28 The Migration of Birds. By T. A. Coward.

36 Spiders. By C. Warburton, M.A.

61 Bees and Wasps. By O. H. Latter, M.A.

46 House Flies. By C. G. Hewitt, D.Sc.

32 Earthworms and their Allies. By F. E. Beddard, F.R.S.

74 The Flea. By H. Russell.

64 The Wanderings of Animals. By H. F. Gadow, F.R.S.

ANTHROPOLOGY

20 The Wanderings of Peoples. By Dr A. C. Haddon, F.R.S.

29 Prehistoric Man. By Dr W. L. H. Duckworth.

GEOLOGY

35 Rocks and their Origins. By Prof. Grenville A. J. Cole.

44 The Work of Rain and Rivers. By T. G. Bonney, Sc.D.

7 The Natural History of Coal. By Dr E. A. Newell Arber.

30 The Natural History of Clay. By Alfred B. Searle.

34 The Origin of Earthquakes. By C. Davison, Sc.D., F.G.S.

62 Submerged Forests. By Clement Reid, F.R.S.

72 The Fertility of the Soil. By E. J. Russell, D.Sc.

BOTANY

5 Plant-Animals: a Study in Symbiosis. By Prof. F. W. Keeble.

10 Plant-Life on Land. By Prof. F. O. Bower, Sc.D., F.R.S.

19 Links with the Past in the Plant-World. By Prof. A. C. Seward, F.R.S.

PHYSICS

52 The Earth. By Prof. J. H. Poynting, F.R.S.

53 The Atmosphere. By A. J. Berry, M.A.

81 The Sun. By Prof. R. A. Sampson, D.Sc., F.R.S.

65 Beyond the Atom. By John Cox, M.A.

55 The Physical Basis of Music. By A. Wood, M.A.

71 Natural Sources of Energy. By Prof. A. H. Gibson, D.Sc.

PSYCHOLOGY

14 An Introduction to Experimental Psychology. By Dr C. S. Myers.

45 The Psychology of Insanity. By Bernard Hart, M.D.

77 The Beautiful. By Vernon Lee.

INDUSTRIAL AND MECHANICAL SCIENCE

31 The Modern Locomotive. By C. Edgar Allen, A.M.I.Mech.E.

56 The Modern Warship. By E. L. Attwood.

17 Aerial Locomotion. By E. H. Harper, M.A., and Allan E. Ferguson, B.Sc.

18 Electricity in Locomotion. By A. G. Whyte, B.Sc.

63 Wireless Telegraphy. By Prof. C. L. Fortescue, M.A.

58 The Story of a Loaf of Bread. By Prof. T. B. Wood, M.A.

47 Brewing. By A. Chaston Chapman, F.I.C.

82 Coal-Mining. By T. C. Cantrill.

83 Leather. By Prof. H. R. Procter.

"A very valuable series of books which combine in a very happy way a popular presentation of scientific truth along with the accuracy of treatment which in such subjects is essential.... In their general appearance, and in the quality of their binding, print, and paper, these volumes are perhaps the most satisfactory of all those which offer to the inquiring layman the hardly earned products of technical and specialist research."--Spectator

"A complete set of these manuals is as essential to the equipment of a good school as is an encyclopaedia.... We can conceive no better series of handy books for ready reference than those represented by the Cambridge Manuals."--School World

Cambridge University Press C. F. Clay, Manager LONDON: Fetter Lane, E.C. EDINBURGH: 100 Princes Street

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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.