Rocks and Their Origins

INTRODUCTION[58]

Igneous rocks, those varied masses that have consolidated from a state of fusion, attracted attention in the eighteenth century through their active appearance in volcanoes. James Hutton in 1785 showed that the crystalline granite of the Scottish highlands "had been made to invade that country in a fluid state." More than a hundred years, however, elapsed before geologists on the continent of Europe were willing to connect superficial lavas with the materials exposed by denudation in consolidated cauldrons of the crust.

It is interesting therefore to note that G. P. Scrope in 1825 treated of granite, without apology or hesitation, in a work entitled "Considerations on Volcanoes." So far from separating deep-seated from superficial products, Scrope wrote of the molten magma in the crust as "the general subterranean bed of lava." He conceived this fundamental magma, "the original or mother-rock," to be capable of consolidating as ordinary granite. Successive meltings and physical modifications of this granite gave rise, in his view, to all the other igneous rocks. Scrope laid no stress, however, on chemical variations within the magma, but urged that the transitions observable between different types of igneous material established a community of origin.

The connexion between lavas and highly crystalline deep-seated rocks, so simply accepted by Scrope, was worked out some fifty years later by J. W. Judd for areas in Hungary and in the Inner Hebrides. The features displayed in thin sections under the microscope were used by Judd, in a series of papers, to substantiate his views; but in France and Germany these features became the source of subtle distinctions between the igneous rocks of Cainozoic and pre-Cainozoic days. The lavas, in which some glassy matter could be traced, were said to be typically post-Cretaceous, and essentially different from those earlier types in which glass was replaced by finely crystalline matter; while the coarsely crystalline igneous rocks were uniformly regarded as pre-Cainozoic. Glassy rocks, such as pitchstone, interbedded contemporaneously in Permian or Devonian strata, were described as "vitreous porphyries," while those known to be of post-Cretaceous date might be styled andesites, trachytes, or rhyolites. Luckily common sense has recently triumphed in this matter, and the relative scarcity of glassy types of igneous rocks in early geological formations has been recognised as due to the readiness with which glass undergoes secondary crystallisation. The discussion has ended by showing that we have no evidence of world-wide changes in the types of material erupted during geological time.

At the present day, attention has been focused on the processes that go on in subterranean cauldrons, in the hope of explaining the differences between one type of extruded rock and another. Doctrines of descent have been elaborated, and one of the most subtle systems of classification[59] has been based upon characters that the igneous rock might have possessed, had circumstances not imparted others to it during the process of consolidation. The principle of this classification is, however, obviously correct, if we wish to trace back a rock bearing certain characters at the present day to the molten source from which it came.

CHARACTERS OF IGNEOUS ROCKS

The characters of igneous rocks vary considerably according as they have consolidated under atmospheric pressure only, or under that of superincumbent rocks. We must remember also that submarine lavas have to sustain a pressure of an extra atmosphere for every thirty feet of depth, or 400 atmospheres at 2000 fathoms, and that such rocks have a claim to be regarded as deep-seated. The gases that igneous rocks contain, probably as essential features of the molten magma, and at a temperature above their critical points, escape to a large extent near or at the surface of the earth. The bubbles raised in lava, whereby it is rendered scoriaceous, and the clouds of vapour rising from cooling lava-flows and from the throat of a volcano in eruption, are sufficient evidences of this process. The extremely liquid lavas of Kilauea in Hawaii, which emit very little vapour, are notable as exceptions. In the case of masses that cool underground, the retention of gases, and ultimately of liquids, until a very late stage of consolidation retards crystallisation until temperatures are reached lower than those at which it starts in surface-flows. As A. Harker points out[60], "the loss of these substances, by raising the melting-points in the magma, may be the immediate cause of crystallisation, quite as much as any actual cooling."

The formation of crystals in lavas is rapid, and the average crystals are therefore small, and often felted together in a mesh, the interstices of which are filled by residual glass.

Slowness of cooling is the really important factor that affects the size of crystals, that is, the coarseness of grain, in igneous rocks. Pressure may promote crystallisation, by raising the melting-points of minerals; but, after a certain maximum effect in this direction, it is quite possible that an increase of pressure may actually lower the melting-points, and cause one or other mineral to remain in solution in the magma. It is not clear how pressure can affect the size of any constituent, except by bringing about conditions under which it can go on growing, while other constituents remain in solution, or do not grow so fast.

Such conditions may arise from the aid given by pressure to the retention of what French geologists have called agents minéralisateurs. Several familiar minerals, for instance albite, orthoclase, and quartz, require the presence of water for their formation. Volatile substances, not utilised in the ultimate product, no doubt similarly assist the formation of many rock-forming minerals. Occasionally, moreover, as in the development of the micas and certain of the silicates known as zeolites, some proportion of hydrogen is retained by minerals thus crystallising from the magma. Micas appear to require the presence of fluorine for their development. J. P. Iddings[61], however, lays stress in this case on the chemical activity of hydrogen at high temperatures.

Igneous rocks, unless cooled with singular rapidity, thus contain crystals of various kinds. In lavas, these may form the globular aggregates known as spherulites[62], or may accumulate as a compact ground of minute grains and needles, not quite resolvable with the microscope. In many rocks of slightly coarser grain, a compact lithoidal or stony texture is set up, which the microscope resolves into an aggregate of crystalline rods or granules. Such compact rocks are often styled felsitic. In other types, as in ordinary granite, the constituent minerals are easily distinguished with the naked eye.

The order in which these constituents have developed is sometimes clear from the inclusion of one mineral in another. When two substances are dissolved in one another, there is a certain proportion between them, varying with the substances, which allows them to crystallise at the same time, instead of in succession. This eutectic proportion, when attained by two mineral substances in a magma, brings about a complete interlocking of their crystals, as is seen in the quartz and alkali-felspar of the rock known as "graphic granite." The order of crystallisation of minerals from an ordinary non-eutectic magma is profoundly affected by the proportions in which their constituents are present in the mass.

The minerals, when they have separated out, are found to be mostly silicates. A few oxides, such as rutile, magnetite, and ilmenite, may occur, the two latter being especially common where iron is an important constituent of the rock. But almost all igneous rocks consist largely of one or more species of felspar, silica being here combined with alumina, potash, soda, and lime. Free silica may remain, and separates as quartz, or rarely as tridymite. Pale mica occurs in many rocks of deep-seated origin. In contrast with these light-coloured minerals, iron, magnesium, and part of the calcium, appear in another series of silicates, usually dark in colour, and this series may be broadly styled "ferromagnesian." The pyroxenes, of which augite is the type, the amphiboles, of which hornblende is the type, dark mica (mostly biotite), and olivine, are the ordinary ferromagnesian minerals.

Broadly, then, igneous rocks divide themselves by texture into (i) those which are completely crystalline, and in which the minerals are distinctly visible; (ii) those which are completely crystalline, but in which the crystals are so small as to give rise to a compact lithoidal ground-mass; and (iii) those in which some glass is present. The third group may appear lithoidal, or in other cases actually glassy, to the unaided eye.

This mode of division is justified from a natural history point of view. The first group includes rocks that have consolidated slowly underground. The second includes rocks cooled more quickly, on the margins of magma-basins, or as offshoots from them, filling cracks in the surrounding rocks, and producing wall-like masses known as dykes. The third group appears mostly in dykes and lava-flows.

Where a dyke has intruded among heated rocks and undergoes no sudden chilling, it may become coarsely crystalline, even though comparatively small. Some dykes exhibit a chilled margin of glass along their bounding surfaces, and are none the less completely crystalline at the centre, where cooling has been slow. No structure is peculiar to dyke-rocks, nor can a class be established for such rocks on chemical or mineralogical grounds, even though a few special types of igneous rock may at present be known only among these minor intrusive bodies.

The fine-grained layers of volcanic dust, commonly spoken of as ash, and the coarser tuffs, in which lumps of scoriaceous lava are clearly visible, bridge the gap between sedimentary and igneous rocks. The dust, during a great eruption, is distributed by wind over hundreds of square miles of country. The tuffs, deposited nearer the orifice of the volcano, vary in coarseness from day to day, and exhibit marked stratification. Ash-beds and tuffs may be laid out in lakes or in the sea, and their layers may then include organic remains. Waves may round their particles on the shore, and may sift them till only a coarse volcanic sand remains.

After an eruption, the newly deposited ash and tuff usually form obvious layers on the surface of the country. Landslips on the side of the volcanic cone may reveal sections of the new coating and of previously stratified material (Fig. 14). In certain districts, sedimentary and other rocks torn off from below form a large part of the fragmental deposits of volcanic action. The characteristic volcanic cone is itself due to the greater accumulation of tuffs and ashes near the vent (Fig. 15).

The loose tuffs formed of scoriæ allow water to percolate easily through them, and a cone of fairly coarse material resists the weather well. The remarkable freshness of the extinct "cinder-cones" of Auvergne was thus long ago explained by Lyell. Surfaces of ash, on the other hand, are easily washed down by rain in the form of dangerous mud-flows, which spread across the lowlands, and give rise to compact clays, shrinking as they dry.

Lava-flows are masses of molten rock that have welled out from the vent, without being torn to pieces by the explosion of the gases that they contained. The rapidity of their flow depends on their chemical composition, on the amount of gases present, and on the temperature at which they are extruded. The more highly siliceous lavas, for a given temperature, are more viscous than those towards the basaltic end of the series, which contain only about 48 per cent. of silica. A lava of considerable fluidity will consolidate in somewhat thin sheets with smooth and ropy surfaces. A less fluid type will become markedly scoriaceous, where the vapours endeavour to escape from it; the rugged crust formed on its upper cooling surface will be broken up by the continued movement of the more liquid mass below, and the blocks thus formed may become rolled over the advancing front of the flow and entombed in the portion that has not yet consolidated.

The surface of ordinary lava-flows remains rough for centuries, and only slowly crumbles down before weathering to form a soil. While tuff-beds provide light and fertile lands, the lava-streams remain marked out among them, as sinuous bands of rock, given over to an irregular growth of woodland. By repeated outflows, lavas tend to fill up the interspaces between the earlier streams, just as these have filled up the hollows in the country over which they spread. A uniform surface thus arises, and lava-plains eventually bury a varied land of hill and dale. Where a number of small vents have opened, perhaps along parallel fissures in the earth, the flooding of the country with igneous rock may lead to an appearance of stratification in masses extending over hundreds of square miles. Sections in the igneous series, however, show that the individual flows dove-tail into and overlap one another, more rapidly than is the case with the lenticular masses that constitute an ordinary sedimentary series.

After the constituents of the lava have begun to crystallise, and when the rock may be considered solid, cracks due to contraction are set up. The upper part of the flow, radiating its heat and parting with its gases into the air above, solidifies comparatively rapidly, and cracks arise without much regularity. Now and then, columnar structure, like that of dried starch, appears on a small scale, the columns starting from various oblique surfaces of cooling, and lying in consequence in various directions in the rock.

J. P. Iddings shows that curvature of the columns will result if one portion of the surface loses heat more rapidly than another. As the contraction-cracks bounding the columns spread inwards, the layer reached by them at any time in the lava will be farther in from a part of the surface where cooling is rapid than it will be from a part where it is slow. Hence the layer in the lava where contractional stresses are producing cracks, i.e. the layer reached at any time by the inner ends of the contraction-columns, will be a curved one, and its curvature will increase as it occupies positions more and more removed from the surface of the lava-flow. The axes of the contraction-columns, as they spread, are perpendicular to this layer, and the columns will thus curve as their development proceeds.

The base of a massive lava-flow, however, cools under much more uniform conditions, and the columns, stretching upwards from the ground and produced by slow contraction, give rise to the regular prismatic structures long ago known as "giants' causeways." The original Giant's Causeway in the county of Antrim is the lower part of a basaltic flow, exposed by denudation on the shore. Fingal's Cave in Staffa owes its tough compact roof to the preservation of that portion of the flow which cooled downwards from the upper surface. G. P. Scrope[63] long ago observed this dual structure in columnar lavas.

The columns, or the more irregular joint-blocks that sometimes represent them, are often subdivided by further contraction into spheroids, the coats of which peel off, as the rock weathers, like those of an onion. The curved cross-joints of massive columns, now convex upwards, now concave, represent the same tendency towards globular contraction.

A lava-flow is sometimes divided into large rudely spheroidal masses, which fit into one another, and which show signs of more rapid cooling on their surfaces. These were particularly observed on the mountains near Mont Genèvre by Cole and Gregory[64], who compared the forms to "pillows or soft cushions pressed upon and against one another." It was suggested that these forms were produced by the seething of viscid lavas, masses being heaved up and falling over, and the outer layers having time to cool in a glassy state before they were deformed by contact with others. This pillow-structure has been widely recognised, and J. J. H. Teall has remarked how often "pillow-lavas" are associated with radiolarian cherts. He regarded them, therefore, as of submarine origin. Sir A. Geikie[65], moreover, stated that the spheroidal sack-like structure was produced by the flow of such lavas into water or watery silt. This acute suggestion has now been verified by Tempest Anderson[66], who has observed in Samoa the chilling of the lobes of lava, as they are thrust off into the sea and washed over by the waves. H. Dewey and J. S. Flett[67] have pointed out that pillow-structure commonly occurs in lavas in which there has been a conversion of lime soda felspars into albite, a change frequent in a series of rocks which they call the "spilitic suite." The importation of soda is attributed to vapours entering soon after the consolidation of the rock, and it is urged that any excess of sodium silicate must have escaped into the sea-water in which the pillow-lavas were produced. Hence radiolaria will flourish in the neighbourhood (presuming that a decomposition of the silicate can be brought about), and their remains will in time form flint in the hollows of the lavas. The paper quoted contains numerous references to previous work, and is a suggestive example of how petrographic study may go hand in hand with the appreciation of rocks from a natural history point of view. It is only characteristic of the subject of petrology that G. Steinmann[68] has with equal ingenuity explained the relations between radiolaria and spilitic lavas by reminding us that gravity-determinations show an excess of basic material under the oceans and of lighter material, rich in silica, under continental land. Hence, when deep-sea deposits are crumpled by earth-movements, basic types of rock, graduating even into serpentine, become associated with radiolarian chert, partly as extruded lavas, but usually as intrusive sheets injected at the epoch of mountain-building.

The characters of igneous rocks in dykes, that is, of those types that have consolidated in fissures, resemble in many respects the characters of lava-flows. Chilling being usually equal on both surfaces, glassy or compact types of rock occur on both sides, and the dyke is, as previously observed, more crystalline in the centre. Columnar structures arise from both surfaces, the dyke also shrinking parallel to its margins. In the outer layers so formed, the columns are small, and they increase in diameter nearer the centre. In small dykes and veins, the columns may run continuously from side to side; in larger ones, they meet along a central surface, which forms, on weathering, a plane of weakness in the rock. Dykes may thus become worn away, decay spreading from the central region, and leaving the more resisting and more glassy portions clinging to the bounding walls.

Where, however, the surrounding rocks are more easily worn away than the igneous invader, as very often happens, the dykes stand out on the surface as great ribs and walls.

The rocks cooled in the deep-seated cauldrons, under what are styled plutonic conditions, have parted with their gases so slowly that they do not show scoriaceous structure. They may become very coarsely crystalline, like many of the Scandinavian granites; minerals, moreover, may be produced which are unstable or difficult to form nearer the surface. Crystals developed in plutonic surroundings become carried forward when the partially consolidated mass is pressed up to a volcanic orifice, and may undergo resorption on the way. Many, however, escape, and impart a porphyritic structure to lavas. The deep-seated rock, from causes that promote the growth of one mineral and the retention of another in solution, may also become "porphyritic" in situ, smaller crystals, or even a eutectic intergrowth, finally filling in the ground.

The viscidity of igneous rocks may cause any of the types to show a fluidal structure. Constituents already formed become dragged along in parallel series as the mass moves forward. Sometimes a group of spherulites, or a knot of "felsitic" matter caused by the dense growth of embryo-crystals, is stretched out into a sheet, and on fractured surfaces a banded structure characterises the mass. These banded rocks record, in their crumpled and obviously fluidal layers, the formerly molten condition of the mass. Even completely crystalline rocks may show parallel arrangement of their minerals, owing to flow during the last stages of consolidation, or to pressure from the walls of the cauldron, influencing the positions taken up by crystals that possess a rod-like or platy form.

The conspicuously banded structures in some crystalline rocks that are often grouped with the metamorphic gneisses may, however, be best explained by their composite origin, and the history of the structure is easily determinable in the field. A common case arises where a granite magma, perhaps already bearing crystals, is intruded, under pressure operating from a distance, into a well-bedded series of sedimentary rocks. The sediments open up like the leaves of a book and admit the invader along their planes of stratification. Even limestone may thus become interlaminated with an igneous rock, just as basalt has been known to separate the annual rings of trees involved in it. This intimate admixture permits of extensive mineral changes, and the two types of rock, probably very different in geological age, become welded together into a composite gneiss, both members of which have influenced one another by contact-metamorphism, often across a wide stretch of country (Fig. 16).

Intrusive igneous rocks in the field will, however, ordinarily prove their character by cutting somewhere across the prevalent structure of the district. When the materials that elsewhere form dykes penetrate between strata for considerable distances as intrusive sheets, they may yet be traced to some point where they have made use of a crack across the bedding. The necks or plugs of old volcanic centres sometimes seem to occupy orifices drilled, or rather shattered, by explosion right through the overlying obstacles. The approximately circular necks in South Africa, filled by brecciated masses of serpentinous rock, are notable examples. The underground cauldrons themselves, when brought to light by denudation, are represented by regions of crystalline rock, which may have various relations to their surroundings. We may trace, in every case, upon their margins the ramifying veins that first proved to James Hutton that granite was younger than the rocks among which it lay. But the portion exposed may be merely the top of a huge body or batholite of igneous matter, stretching far down into the crust; or it may be part of a localised knot, which filled up some cavity provided for it by earth-movement, oozing in step by step as room was made for its advance. In the latter case, it was originally bounded above by some series of strata which was arched up as a dome or as an anticline. Or possibly strata have been moved apart from one another, the upper ones sliding over the lower ones and at the same time bulging upwards, so as to leave a cavity of roughly hemispherical form. Such a space, allowing relief from pressure, will be occupied by igneous rock, which may or may not have a direct root through the stratum underneath it. The igneous mass may in such cases be merely an expansion of a large intrusive sheet. It sends off veins into the roof above, and can only be distinguished from a batholite by the presence of stratified rock beneath it. Occurrences of this kind were first described in the Henry Mountains of Utah by G. K. Gilbert, who gave them the name of "stone-cisterns" or laccoliths, a word now commonly written laccolites. It may be questioned if the expansion of the gases in the intruding igneous rock is sufficient in itself to form the laccolitic dome. The igneous rock has probably been pressed into position by the forces that produced the earth-movements.

In many cases, batholites seem to have worked their way upwards without any relation to earth-movements in the district. The processes by which they come into place among other rocks are worthy of separate consideration.

THE INTRUSION OF LARGE BODIES OF IGNEOUS ROCK

Attention has been already called to the composite gneisses formed by the intrusion of an igneous magma between the leaves, as it were, of sediments. Such occurrences are often seen on the margins of batholites or of any kind of igneous dome, and they no doubt represent the picking off of layer after layer from the walls surrounding the intrusive mass. If these layers can become absorbed into the igneous rock, the crest of the dome can advance, and the dome itself can widen, so long as sufficient heat is supplied to it from below. Space is found for the intrusive mass at the expense of the marginal rocks; but it is obvious that the portions absorbed merely add to the bulk of the igneous material. The composition of the latter must also undergo modification. Its great size, reaching as it does far down into the crust, in comparison with the quantity of matter absorbed in the upper regions, may render such modification very difficult to trace beyond the latest zone of contact.

Petrologists differ very widely as to the extent to which igneous masses assume their place in the upper regions of the crust by processes of "stoping," absorption, and assimilation. The statement, however, in a recent work that "the assimilation hypothesis" is "still supported by some French geologists" is calculated to surprise those who recognise the trend of modern opinion both in America and on the continent of Europe. Far from the views of A. Michel Lévy, C. Barrois, and A. Lacroix, surviving as an expression of national perversity, they have been supported to a remarkable degree by the observations of Sederholm in Finland, of Lepsius and H. Credner in Saxony, of A. Lawson and F. D. Adams in North America, and by the careful reasoning of C. Doelter[69] based largely on his own experimental work. A. Harker[70] and J. P. Iddings[71] have argued that assimilation is merely a local phenomenon, of little importance in the theory of igneous intrusion. W. C. Brögger[72], however, who strongly supports the laccolitic view for the Christiania district, expresses himself with far more caution, and leaves the way clear for conclusions as to absorption and mingling of molten products in the lower regions of the crust.

Doelter lays stress on the influence of high temperature, and especially of the highly heated gases in the igneous rock, in promoting corrosion of the cauldron-walls. He attributes greater power of corrosion to the magmas rich in silica, and agrees with R. A. Daly that the rapidly moving basic magmas reach the upper layers of the crust in a condition of comparative purity. Daly[73] may be looked on as an extremist in this matter; but it is hard for those who have studied regions where the deep-seated cauldrons have been cut across by denudation to avoid very large views of igneous absorption. The contact-zones between the igneous mass and the surrounding rocks are often seen merely in cross-section on the flanks of a batholite or laccolite. In the areas of Archæan rocks, on the other hand, where prolonged denudation has exposed the zones of repeated interaction over hundreds of square miles on an approximately horizontal surface, one may form some idea of the processes that are still effective in the depths.

G. V. Hawes[74], in 1881, recognised the importance of the process known by the mining term of "stoping," as a means whereby igneous rocks work their way upward in the crust. Cracks in the overlying roof are entered by the magma, blocks are wedged off, and these are ultimately absorbed in the molten mass. In this matter Hawes stands as a pioneer. As the viscosity of the magma increases during cooling, the blocks last detached may remain embedded in the marginal zone. The remarkable purity of this zone, however, in many cases has raised an obvious difficulty; but it has been pointed out[75] that the modified marginal and composite rock may continuously sink down into the depths, aided by any of the causes that promote magmatic differentiation, while a fairly pure magma, almost of the original composition, is left on the crest of the advancing dome. R. A. Daly[76] has developed the stoping theory with considerable boldness. The areas most likely to carry conviction to those who doubt that igneous masses can be intruded at the expense of their surroundings are those where banded gneisses have arisen on a regional scale (see p. 160).

THE RANGE OF COMPOSITION IN IGNEOUS ROCKS

The broad division of igneous rocks into those of light colour and of low specific gravity on the one hand and those that are dark and heavy on the other is a very natural one, and Bunsen and Durocher insisted that two magmas were fundamental in the crust. In one of these, the "acid" magma, which gives rise to granites and rhyolites, silica formed about 70 per cent. by weight of the ultimate rocks; in the other, it formed about 50 per cent., and the products are basic diorites, gabbros, and basalts[77]. The former group of rocks is rich in alkalies, the latter, the "basic" group, in calcium, magnesium, and iron. The mixture of these more extreme types of magma was held to give rise to what are now called "intermediate" rocks.

Two other views are of course possible. If the composition of the globe was originally uniform, the two magmas must have arisen by separation from one of intermediate nature. Hence, in any cauldron in the crust, in place of one of two magmas, an intermediate magma may be presumed to exist, and to split up, from various causes, into a number of parts which are separately erupted at the surface. Charles Darwin's[78] remarks as to the sinking of crystals in a cooling magma, and the consequent production of a trachytic and basaltic type in the same cauldron, led the way to a general acceptance of the theory of magmatic differentiation in laccolites and batholites. W. C. Brögger's[79] brilliant explanation of the variation and succession of types of igneous rock in the Christiania district has had a profound influence on workers in other fields, and has perhaps directed attention away from the parallel possibilities of differentiation by assimilation.

The assimilation theory provides the second possible view above referred to. A magma may be modified by the rocks into which it intrudes, so that a "basic" fluid may become charged with silica from a sandstone, the product crystallising as a granite; while an "acid" fluid may become so charged with limestone that diorite ultimately results. A. Harker[80] has discussed both theories clearly, with a strong leaning to the acceptance of magmatic differentiation in the cauldron as the only important cause of variation. R. A. Daly, on the other hand, goes at least as far as Lacroix in France in supporting the theory of assimilation. For him, the primitive igneous magma is already basic, and basalts are therefore the prevalent type of igneous rock. They reach us, moreover, from considerable depths. The acid rocks are formed by amalgamation of this magma with siliceous material lying nearer the earth's surface. Igneous rocks exceptionally rich in alkalies, the so-called "alkaline" series, result from the absorption of limestone in the magma; denser lime-bearing silicates are thus formed, which sink by gravitation, leaving a lighter magma above in which soda has become concentrated. Carbon dioxide liberated from the limestone also plays a part in carrying up the alkalies that might otherwise remain in a lower portion[81].

E. H. L. Schwarz[82] extends Daly's views with an almost romantic fulness. He holds, with Chamberlin, that the primitive globe resulted from the aggregation of basic meteoritic material. The more siliceous crust arose from the withdrawal of magnesium and iron into the depths by long-continued processes of leaching and gravitation. The melting of this crust produces the acid igneous rocks. Igneous cauldrons originate in the heat due to faulting, or to crumpling, or even to the impact of gigantic meteorites. When a molten magma is locally established, variation occurs in it by assimilation of different types of material round it.

The balance of judgment as to differentiation and assimilation, which should be regarded as parallel probabilities rather than as rival propositions, is admirably held by C. Doelter[83], whose chapters on this matter can be appreciated by all geologists.

It is of course possible that differentiation of type, from various causes, has already proceeded so far in the earth's crust as to produce noteworthy contrasts in the rocks erupted in different areas. The interior of our globe, on Chamberlin's planetesimal hypothesis, need not have been uniform in constitution, either at the outset or at any subsequent time. J. W. Judd[84] has called attention to the existence of petrographical provinces, a conception that has been very fruitful in results. These provinces have been grouped by Harker[85] in two branches, characterised respectively by rocks rich in alkalies and by rocks rich in lime. The former branch appears to be associated with the movements of faulting and block-structure, rather than of crumpling, that have produced E. Suess's "Atlantic" type of coast. The rocks rich in lime, on the other hand, are said to be characteristic of areas that have been folded like the countries bordering the Pacific. The names "Atlantic" and "Pacific" have consequently been given to the two branches, but these terms seem too geographical in their suggestion. Dewey and Flett[86] have put forward a third type of magma, giving rise especially to albite as a primary or secondary constituent, and characterised by the production of pillow-lavas. This type is held to be associated with areas that have steadily subsided, without much folding. G. Steinmann[87], however, has connected the spilites and "ophiolitic" rocks with regions of intense over-folding.

So far, there are many cases where it is difficult to assign a petrographic province to one or other of these branches, and the system seems to demand more simplicity within the provinces than nature is prepared to yield.

Whatever the causes of variation, it is necessary to mark out by names certain kinds of igneous material, and it is generally accepted that the types thus set up are best based on chemical composition. At the same time, the minerals present in the rock, and also its structure, record certain phases of its history, and deserve an important place in any system of classification. The natural history of an igneous rock is concerned with its mode of occurrence, and no isolated specimen can satisfy the geological investigator. In the field, the porphyritic crystals, which have an important influence on the total chemical composition, may be found to be strangers to the magma, and to have been derived from some mass imperfectly absorbed. The dark flecks and patches in a granitoid rock, so often ascribed, somewhat mysteriously, to local "segregation" in the magma, again and again prove to be metamorphosed and minutely injected fragments of foreign rocks[88].

None the less, a broad classification is possible on chemical grounds, and the acid, intermediate, basic, and ultrabasic grouping adopted by Judd has been found of great convenience. Among acid rocks we have granite as the coarsely crystalline type, with potassium felspars prevalent and the excess of silica manifest as quartz. The finer grained and sometimes compact types are the eurites, quartz-felsites, or quartz-porphyries. When the rock contains more or less residual glass, we have what are now known as rhyolites, of which ordinary obsidian is the most glassy representative.

The opposite types, those of the basic group, include, at the coarsely crystalline end, gabbro and basic diorite; the finely crystalline forms are styled dolerites, and those with a trace of glass, or at any rate very fine-grained and compact, are basalts. Glassy types are naturally rare in this group, owing to the unsuitable chemical composition.

Between granite and gabbro lie various rocks of intermediate composition, some of them rich in soda rather than in potash. Syenite, granodiorite, and the diorites with a prevalence of soda over lime, are coarsely crystalline types. Compact types of these of course occur. It will be sufficient, however, here to name the forms with traces of residual glass, which range from trachyte, the type rich in potash, to andesite, which connects them with basalt, in a series where lime ultimately predominates over soda.

In the ultrabasic group are a number of exceptional types. Olivine often becomes an important constituent, and the rocks then decompose into the soft green or reddish masses known as serpentine—or, more properly, serpentine-rock.

Igneous rocks, owing to their range of mineral composition and of structure, combined with their general hardness, lend themselves to various economic purposes. While the granites, resisting atmospheric attack admirably in a polished state, provide our handsomest building-stones, dolerites and fine-grained diorites, which owe their toughness largely to the interlocked relations of their constituent minerals, serve as our most satisfactory road-metals.

THE SCENERY OF IGNEOUS ROCKS

Volcanic landscapes, where activity is very recent or still in progress, present a number of characteristic surface-forms. The cones that have accumulated round the vents surpass all other hills in regularity of outline, and the crater in the summit is often relatively large. Lava-cones may be steep-sided bosses when formed of protrusions of viscid rocks rich in silica, like the remarkable domes in the north of Bohemia, or they may present very gentle slopes where fluid basic lavas have been extruded.

Tuff-cones are liable to be breached on one side, owing to the outflow of lava which the crater-wall could not sustain, and they then assume the form of a mountain in which glacial influences have hollowed out a cirque.

Rain washes down the loose materials from great volcanic cones, and emphasises the concave curve of the mountain sides, the form that is so beautiful in Fuji-yama in Japan, and which Hokusai, with pardonable and affectionate exaggeration, reproduced in a hundred illustrations. Ultimately, however, grooves appear on the flanks of the cone, in which permanent streams gather, and the slopes are dissected and worn away. During this process, the tuffs yield steep and fantastic forms, and wall-like dykes weather out among them. The dykes are usually the last features to decay.

Where the vent has been plugged with lava at the close of its activity, the neck of rock often remains standing above the surrounding country. The site of cone after cone can be picked out in this way in the Cainozoic volcanic areas of central Germany. The jutting crag of trachyte or of basalt has often been seized on as the site of a feudal castle, under which the dependent agriculturists still gather at nightfall in their red-roofed town. The group of sheer-sided necks in the Hegau in southern Württemberg, the Hohentwiel, Hohenkrähen, and the rest, form a very striking landscape amid undulating Cainozoic lands.

The lava-beds that cover wide areas are naturally of basic composition. Basalts thus form enormous plains with rugged surfaces, on which at last a red-brown soil collects. When exposed to denudation from the edge of the region inwards, they develop a marked terrace-structure, through which the rivers cut steep and grim ravines. Grass may grow on the ledges and the tables; but the scarps, controlled by the well-marked vertical jointing of the lavas, remain sharp and prominent, and the rock falls away from these walls in whole columns at a time. This structure is characteristically seen in northern Mull and the adjacent smaller isles, and is still more impressive from the centre to the north of Skye, where the rain swept terraces covered by grass and bog and scanty oatfields, and the black steps of rock between them, present a scene of strange monotony and desolation.

In regions less exposed to stormy weather, the lava-plateaus may provide good soils. For instance, after the great seaward scarp of the basalts has been crossed in the counties of Antrim and of Londonderry, the lava-fields, dropped by faults towards Lough Neagh, are seen to be occupied by prosperous farms. In arid countries, however, the savage surface of the flows merely becomes modified by red dust and scoriaceous gravel, worn by wind and changes of temperature from the upstanding portions of the land.

Where a stratified country has been freely invaded by sheets of lava along its planes of bedding, the stratification is emphasised in any part exposed to weathering. The resisting igneous rock stands out in scarps along the hills, and marks out any folds that have been formed since the epoch of its intrusion.

When the beds remain fairly level, and are also uplifted, flat-topped hills are formed by the intrusive sheets, like those that may be carved out of a country flooded over by lava-streams. The crystalline rock, very probably a dolerite, protects what lies below it. The kopjes north of the Great Karroo in the centre of the Cape of Good Hope are thus level on the crest and bounded by a steep wall or krans of rock.

The edges of similar "sills" of igneous rock have controlled much of the scenery between the Highland border of Scotland and the Tyne. A fine example is the indented scarp of the Great Whin Sill, a sheet of dolerite intruded among the Carboniferous strata of Northumberland. This mass forms a platform for Bamburgh Castle against the wild North Sea, and is traceable south-westward across the country towards Carlisle. North of Hexham, its escarpment is occupied by Hadrian's wall, and the town of Borcovicus was planted on the edge, overlooking all Northumbria.

The farmers of North Britain and Ireland have long known upstanding igneous dykes as unprofitable "whinstones." The regularity of direction among dykes over very wide areas points to their intrusion along cracks produced by stretching of the crust. Radial grouping of dykes, such as one finds near volcanic necks, or, on a gigantic scale, round Tycho on the moon, may be due to explosive action; but the majority of dykes seem to have followed upon earth-movement. In the north of Ireland, from the coast of Down to that of Donegal, a series of compact rocks of Devonian age occurs in dykes lying almost invariably north and south. The post-Cretaceous dykes of the same region have a still more uniform trend, from north-west to south-east. Such series of dykes modify the scenery of coasts by forming promontories and serviceable piers for boats.

The offshoots near the surface of a great intrusive mass are far less regular. We are here close to the zone of attack, the "shatter-zone," and the structures or regular fracture-planes of the overlying rock only partially control the position taken up by the intrusive magma. Irregular knots and bosses appear in place of far-spreading sheets, and a network of crossing veins occurs, instead of a system of co-ordinated dykes. The resulting country is hummocky and broken, and, where the cauldron itself has become exposed, striking contrasts of surface are seen as we pass from the igneous core to the older and frequently stratified rocks upon its flanks.

Some large bodies of intrusive rock have, however, been formed sheet by sheet, and a bedded sill-like structure is then revealed in them on weathering. Sir A. Geikie[89] calls attention to this in his description of the heart of the black gabbro mass in Skye. But, as a rule, the continuity of structure in batholites, and their characteristic joint-planes set at angles to one another, cause them to appear as massive blocks in the landscape, untraversed by any regular lines.

Granite, with its broad tabular jointing, which is often developed parallel to a surface of cooling, forms rounded slopes and domes after long-continued weathering. When reared high into the zone of frost-action, it develops spires and pinnacles, as in the huge "aiguilles" of Mont Blanc. But, as decay goes on, the uniform descent of boulders and sand forms spreading taluses, banked against the lower slopes, while the curving joints, not too closely set, promote a smoothness on the higher lands. These joints, moreover, divide the rock into boulders almost ready-made. Tabular structure sometimes predominates; but even in this case the exposed ends of the layers soon become rounded, as the felspar crystals pass into a powdery state. Commonly, a rough spheroidal structure prevails, as may be traced in many of the Dartmoor "tors," and the blocks that slip away through widening of the joints become more and more rounded as their surfaces crumble on the talus (Fig. 17).