A Manual of Elementary Geology Part 60

[Ill.u.s.tration: Fig. 445. Greenstone dike, with fragments of gneiss.

Sorgenfri, Christiania.]

The fact above alluded to, of a foreign fragment, such as _b_, fig.

444., included in the midst of the trap, as if torn off from some subjacent rock or the walls of a fissure, is by no means uncommon. A fine example is seen in another dike of greenstone, 10 feet wide, in the northern suburbs of Christiania, in Norway, of which the annexed figure is a ground plan. The dike pa.s.ses through shale, known by its fossils to belong to the Silurian series. In the black base of greenstone are angular and roundish pieces of gneiss, some white, others of a light flesh-colour, some without lamination, like granite, others with laminae, which, by their various and often opposite directions, show that they have been scattered at random through the matrix. These imbedded pieces of gneiss measure from 1 to about 8 inches in diameter.

_Rocks altered by volcanic dikes._--After these remarks on the form and composition of dikes themselves, I shall describe the alterations which they sometimes produce in the rocks in contact with them. The changes are usually such as the intense heat of melted matter and the entangled gases might be expected to cause.

_Plas-Newydd._--A striking example, near Plas-Newydd, in Anglesea, has been described by Professor Henslow.[381-A] The dike is 134 feet wide, and consists of a rock which is a compound of felspar and augite (dolerite of some authors). Strata of shale and argillaceous limestone, through which it cuts perpendicularly, are altered to a distance of 30, or even, in some places, to 35 feet from the edge of the dike. The shale, as it approaches the trap, becomes gradually more compact, and is most indurated where nearest the junction. Here it loses part of its schistose structure, but the separation into parallel layers is still discernible. In several places the shale is converted into hard porcellanous jasper. In the most hardened part of the ma.s.s the fossil sh.e.l.ls, princ.i.p.ally _Producti_, are nearly obliterated; yet even here their impressions may frequently be traced. The argillaceous limestone undergoes a.n.a.logous mutations, losing its earthy texture as it approaches the dike, and becoming granular and crystalline. But the most extraordinary phenomenon is the appearance in the shale of numerous crystals of a.n.a.lcime and garnet, which are distinctly confined to those portions of the rock affected by the dike.[382-A] Some garnets contain as much as 20 per cent. of lime, which they may have derived from the decomposition of the fossil sh.e.l.ls or Producti. The same mineral has been observed, under very a.n.a.logous circ.u.mstances, in High Teesdale, by Professor Sedgwick, where it also occurs in shale and limestone, altered by basalt.[382-B]

_Antrim._--In several parts of the county of Antrim, in the north of Ireland, chalk with flints is traversed by basaltic dikes. The chalk is there converted into granular marble near the basalt, the change sometimes extending 8 or 10 feet from the wall of the dike, being greatest near the point of contact, and thence gradually decreasing till it becomes evanescent. "The extreme effect," says Dr. Berger, "presents a dark brown crystalline limestone, the crystals running in flakes as large as those of coa.r.s.e primitive (_metamorphic_) limestone; the next state is saccharine, then fine grained and arenaceous; a compact variety, having a porcellanous aspect and a bluish-grey colour, succeeds: this, towards the outer edge, becomes yellowish-white, and insensibly graduates into the unaltered chalk.

The flints in the altered chalk usually a.s.sume a grey yellowish colour."[382-C] All traces of organic remains are effaced in that part of the limestone which is most crystalline.

[Ill.u.s.tration: Fig. 446. Basaltic dikes in chalk in island of Rathlin, Antrim. Ground plan, as seen on the beach. (Conybeare and Buckland.[382-D])]

The annexed drawing (fig. 446.) represents three basaltic dikes traversing the chalk, all within the distance of 90 feet. The chalk contiguous to the two outer dikes is converted into a finely granular marble, _m m_, as are the whole of the ma.s.ses between the outer dikes and the central one. The entire contrast in the composition and colour of the intrusive and invaded rocks, in these cases, renders the phenomena peculiarly clear and interesting.

Another of the dikes of the north-east of Ireland has converted a ma.s.s of red sandstone into hornstone.[382-E] By another, the slate clay of the coal measures has been indurated, and has a.s.sumed the character of flinty slate[383-A]; and in another place the slate clay of the lias has been changed into flinty slate, which still retains numerous impressions of ammonites.[383-B]

It might have been antic.i.p.ated that beds of coal would, from their combustible nature, be effected in an extraordinary degree by the contact of melted rock. Accordingly, one of the greenstone dikes of Antrim, on pa.s.sing through a bed of coal, reduces it to a cinder for the s.p.a.ce of 9 feet on each side.[383-C]

At c.o.c.kfield Fell, in the north of England, a similar change is observed.

Specimens taken at the distance of about 30 yards from the trap are not distinguishable from ordinary pit coal; those nearer the dike are like cinders, and have all the character of c.o.ke; while those close to it are converted into a substance resembling soot.[383-D]

As examples might be multiplied without end, I shall merely select one or two others, and then conclude. The rock of Stirling Castle is a calcareous sandstone, fractured and forcibly displaced by a ma.s.s of greenstone which has evidently invaded the strata in a melted state. The sandstone has been indurated, and has a.s.sumed a texture approaching to hornstone near the junction. In Arthur's Seat and Salisbury Craig, near Edinburgh, a sandstone which comes in contact with greenstone is converted into a jaspideous rock.[383-E]

The secondary sandstones in Skye are converted into solid quartz in several places, where they come in contact with veins or ma.s.ses of trap; and a bed of quartz, says Dr. MacCulloch, found near a ma.s.s of trap, among the coal strata of Fife, was in all probability a stratum of ordinary sandstone, having been subsequently indurated and turned into quartzite by the action of heat.[383-F]

But although strata in the neighbourhood of dikes are thus altered in a variety of cases, shale being turned into flinty slate or jasper, limestone into crystalline marble, sandstone into quartz, coal into c.o.ke, and the fossil remains of all such strata wholly and in part obliterated, it is by no means uncommon to meet with the same rocks, even in the same districts, absolutely unchanged in the proximity of volcanic dikes.

This great inequality in the effects of the igneous rocks may often arise from an original difference in their temperature, and in that of the entangled gases, such as is ascertained to prevail in different lavas, or in the same lava near its source and at a distance from it. The power also of the invaded rocks to conduct heat may vary, according to their composition, structure, and the fractures which they may have experienced, and perhaps, also, according to the quant.i.ty of water (so capable of being heated) which they contain. It must happen in some cases that the component materials are mixed in such proportions as prepare them readily to enter into chemical union, and form new minerals; while in other cases the ma.s.s may be more h.o.m.ogeneous, or the proportions less adapted for such union.

We must also take into consideration, that one fissure may be simply filled with lava, which may begin to cool from the first; whereas in other cases the fissure may give pa.s.sage to a current of melted matter, which may ascend for days or months, feeding streams which are overflowing the country above, or are ejected in the shape of scoriae from some crater. If the walls of a rent, moreover, are heated by hot vapour before the lava rises, as we know may happen on the flanks of a volcano, the additional caloric supplied by the dike and its gases will act more powerfully.

[Ill.u.s.tration: Fig. 447. Trap interposed between displaced beds of limestone and shale, at White Force, High Teesdale, Durham.

(Sedgwick.[384-A])]

_Intrusion of trap between strata._--In proof of the mechanical force which the fluid trap has sometimes exerted on the rocks into which it has intruded itself, I may refer to the Whin-Sill, where a ma.s.s of basalt, from 60 to 80 feet in height, represented by _a_, fig. 447., is in part wedged in between the rocks of limestone, _b_, and shale, _c_, which have been separated from the great ma.s.s of limestone and shale, _d_, with which they were united.

The shale in this place is indurated; and the limestone, which at a distance from the trap is blue, and contains fossil corals, is here converted into granular marble without fossils.

Ma.s.ses of trap are not unfrequently met with intercalated between strata, and maintaining their parallelism to the planes of stratification throughout large areas. They must in some places have forced their way laterally between the divisions of the strata, a direction in which there would be the least resistance to an advancing fluid, if no vertical rents communicated with the surface, and a powerful hydrostatic pressure was caused by gases propelling the lava upwards.

_Columnar and globular structure._--One of the characteristic forms of volcanic rocks, especially of basalt, is the columnar, where large ma.s.ses are divided into regular prisms, sometimes easily separable, but in other cases adhering firmly together. The columns vary in the number of angles, from three to twelve; but they have most commonly from five to seven sides.

They are often divided transversely, at nearly equal distances, like the joints in a vertebral column, as in the Giant's Causeway, in Ireland. They vary exceedingly in respect to length and diameter. Dr. MacCulloch mentions some in Skye which are about 400 feet long; others, in Morven, not exceeding an inch. In regard to diameter, those of Ailsa measure 9 feet, and those of Morven an inch or less.[385-A] They are usually straight, but sometimes curved; and examples of both these occur in the island of Staffa.

In a horizontal bed or sheet of trap the columns are vertical; in a vertical dike they are horizontal. Among other examples of the last-mentioned phenomenon is the ma.s.s of basalt, called the Chimney, in St.

Helena (see fig. 448.), a pile of hexagonal prisms, 64 feet high, evidently the remainder of a narrow dike, the walls of rock which the dike originally traversed having been removed down to the level of the sea. In fig. 449. a small portion of this dike is represented on a less reduced scale.[385-B]

[Ill.u.s.tration: Fig. 448. Volcanic dike composed of horizontal prisms.

St. Helena.]

[Ill.u.s.tration: Fig. 449. Small portion of the d.y.k.e in Fig. 448.]

[Ill.u.s.tration: Fig. 450. Lava of La Coupe d'Ayzac, near Antraigue, in the province of Ardeche.]

It being a.s.sumed that columnar trap has consolidated from a fluid state, the prisms are said to be always at right angles to the _cooling surfaces_.

If these surfaces, therefore, instead of being either perpendicular, or horizontal, are curved, the columns ought to be inclined at every angle to the horizon; and there is a beautiful exemplification of this phenomenon in one of the valleys of the Vivarais, a mountainous district in the South of France, where, in the midst of a region of gneiss, a geologist encounters unexpectedly several volcanic cones of loose sand and scoriae. From the crater of one of these cones called La Coupe d'Ayzac, a stream of lava descends and occupies the bottom of a narrow valley, except at those points where the river Volant, or the torrents which join it, have cut away portions of the solid lava. The accompanying sketch (fig. 450.) represents the remnant of the lava at one of the points where a lateral torrent joins the main valley of the Volant. It is clear that the lava once filled the whole valley up to the dotted line _d a_; but the river has gradually swept away all below that line, while the tributary torrent has laid open a transverse section; by which we perceive, in the first place, that the lava is composed, as usual in this country, of three parts: the uppermost, at _a_, being scoriaceous; the second, _b_, presenting irregular prisms; and the third, _c_, with regular columns, which are vertical on the banks of the Volant, where they rest on a horizontal base of gneiss, but which are inclined at an angle of 45 at _g_, and then horizontal at _f_, their position having been every where determined, according to the law before mentioned, by the concave form of the original valley.

[Ill.u.s.tration: Fig 451. Columnar basalt in the Vicentin. (Fortis.)]

In the annexed figure (451.) a view is given of some of the inclined and curved columns which present themselves on the sides of the valleys in the hilly region north of Vicenza, in Italy, and at the foot of the higher Alps.[386-A] Unlike those of the Vivarais, last mentioned, the basalt of this country was evidently submarine, and the present valleys have since been hollowed out by denudation.

The columnar structure is by no means peculiar to the trap rocks in which hornblende or augite predominate; it is also observed in clinkstone, trachyte, and other felspathic rocks of the igneous cla.s.s, although in these it is rarely exhibited in such regular polygonal forms.

[Ill.u.s.tration: Fig. 452. Basaltic pillars of the Kasegrotte, Bertrich-Baden, half way between Treves and Coblentz. Height of grotto, from 7 to 8 feet.]

It has been already stated that basaltic columns are often divided by cross joints. Sometimes each segment, instead of an angular, a.s.sumes a spheroidal form, so that a pillar is made up of a pile of b.a.l.l.s, usually flattened, as in the Cheese-grotto at Bertrich-Baden, in the Eifel, near the Moselle (fig. 452.). The basalt, there, is part of a small stream of lava, from 30 to 40 feet thick, which has proceeded from one of several volcanic craters, still extant, on the neighbouring heights. The position of the lava bordering the river in this valley might be represented by a section like that already given at fig. 450. p. 385., if we merely supposed inclined strata of slate and the argillaceous sandstone called greywacke to be subst.i.tuted for gneiss.

In some ma.s.ses of decomposing greenstone, basalt, and other trap rocks, the globular structure is so conspicuous that the rock has the appearance of a heap of large cannon b.a.l.l.s.

[Ill.u.s.tration: Fig. 453. Globiform pitchstone. Chiaja di Luna, Isle of Ponza. (Scrope.)]

A striking example of this structure occurs in a resinous trachyte or pitchstone-porphyry in one of the Ponza islands, which rise from the Mediterranean, off the coast of Terracina and Gaeta. The globes vary from a few inches to three feet in diameter, and are of an ellipsoidal form (see fig. 453.). The whole rock is in a state of decomposition, "and when the b.a.l.l.s," says Mr. Scrope, "have been exposed a short time to the weather, they scale off at a touch into numerous concentric coats, like those of a bulbous root, inclosing a compact nucleus. The laminae of this nucleus have not been so much loosened by decomposition; but the application of a ruder blow will produce a still further exfoliation."[387-A]

A fissile texture is occasionally a.s.sumed by clinkstone and other trap rocks, so that they have been used for roofing houses. Sometimes the prismatic and slaty structure is found in the same ma.s.s. The causes which give rise to such arrangements are very obscure, but are supposed to be connected with changes of temperature during the cooling of the ma.s.s, as will be pointed out in the sequel. (See Chaps. x.x.xV. and x.x.xVI.)

_Relation of Trappean Rocks to the products of active Volcanos._

When we reflect on the changes above described in the strata near their contact with trap dikes, and consider how great is the a.n.a.logy in composition and structure of the rocks called trappean and the lavas of active volcanos, it seems difficult at first to understand how so much doubt could have prevailed for half a century as to whether trap was of igneous or aqueous origin. To a certain extent, however, there was a real distinction between the trappean formations and those to which the term volcanic was almost exclusively confined. The trappean rocks first studied in the north of Germany, and in Norway, France, Scotland, and other countries, were either such as had been formed entirely under deep water, or had been injected into fissures and intruded between strata, and which had never flowed out in the air, or over the bottom of a shallow sea. When these products, therefore, of submarine or subterranean igneous action were contrasted with loose cones of scoriae, tuff, and lava, or with narrow streams of lava in great part scoriaceous and porous, such as were observed to have proceeded from Vesuvius and Etna, the resemblance seemed remote and equivocal. It was, in truth, like comparing the roots of a tree with its leaves and branches, which, although they belong to the same plant, differ in form, texture, colour, mode of growth, and position. The external cone, with its loose ashes and porous lava, may be likened to the light foliage and branches, and the rocks concealed far below, to the roots. But it is not enough to say of the volcano,

"quantum vertice in auras aetherias, tantum radice in Tartara tendit,"

for its roots do literally reach downwards to Tartarus, or to the regions of subterranean fire; and what is concealed far below, is probably always more important in volume and extent than what is visible above ground.

[Ill.u.s.tration: Fig. 454. Strata intersected by a trap dike, and covered with alluvium.]

We have already stated how frequently dense ma.s.ses of strata have been removed by denudation from wide areas (see Chap. VI.); and this fact prepares us to expect a similar destruction of whatever may once have formed the uppermost part of ancient submarine or subaerial volcanos, more especially as those superficial parts are always of the lightest and most perishable materials. The abrupt manner in which dikes of trap usually terminate at the surface (see fig. 454.), and the water-worn pebbles of trap in the alluvium which covers the dike, prove incontestably that whatever was uppermost in these formations has been swept away. It is easy, therefore, to conceive that what is gone in regions of trap may have corresponded to what is now visible in active volcanos.

It will be seen in the following chapters, that in the earth's crust there are volcanic tuffs of all ages, containing marine sh.e.l.ls, which bear witness to eruptions at many successive geological periods. These tuffs, and the a.s.sociated trappean rocks, must not be compared to lava and scoriae which had cooled in the open air. Their counterparts must be sought in the products of modern submarine volcanic eruptions. If it be objected that we have no opportunity of studying these last, it may be answered, that subterranean movements have caused, almost everywhere in regions of active volcanos, great changes in the relative level of land and sea, in times comparatively modern, so as to expose to view the effects of volcanic operations at the bottom of the sea.

Thus, for example, the recent examination of the igneous rocks of Sicily, especially those of the Val di Noto, has proved that all the more ordinary varieties of European trap have been there produced under the waters of the sea, at a modern period; that is to say, since the Mediterranean has been inhabited by a great proportion of the existing species of testacea.

These igneous rocks of the Val di Noto, and the more ancient trappean rocks of Scotland and other countries, differ from subaerial volcanic formations in being more compact and heavy, and in forming sometimes extensive sheets of matter intercalated between marine strata, and sometimes stratified conglomerates, of which the rounded pebbles are all trap. They differ also in the absence of regular cones and craters, and in the want of conformity of the lava to the lowest levels of existing valleys.

It is highly probable, however, that insular cones did exist in some parts of the Val di Noto: and that they were removed by the waves, in the same manner as the cone of Graham island, in the Mediterranean, was swept away in 1831, and that of Nyoe, off Iceland, in 1783.[389-A] All that would remain in such cases, after the bed of the sea has been upheaved and laid dry, would be dikes and shapeless ma.s.ses of igneous rock, cutting through sheets of lava which may have spread over the level bottom of the sea, and strata of tuff, formed of materials first scattered far and wide by the winds and waves, and then deposited. Trap conglomerates also, to which the action of the waves must give rise during the denudation of such volcanic islands, will emerge from the deep whenever the bottom of the sea becomes land.

The proportion of volcanic matter which is originally submarine must always be very great, as those volcanic vents which are not entirely beneath the sea, are almost all of them in islands, or, if on continents, near the sh.o.r.e. This may explain why extended sheets of trap so often occur, instead of narrow threads, like lava streams. For, a mult.i.tude of causes tend, near the land, to reduce the bottom of the sea to a nearly uniform level,--the sediment of rivers,--materials transported by the waves and currents of the sea from wasting cliffs,--showers of sand and scoriae ejected by volcanos, and scattered by the wind and waves. When, therefore, lava is poured out on such a surface, it will spread far and wide in every direction in a liquid sheet, which may afterwards, when raised up, form the tabular capping of the land.

As to the absence of porosity in the trappean formations, the appearances are in a great degree deceptive, for all amygdaloids are, as already explained, porous rocks, into the cells of which mineral matter, such as silex, carbonate of lime, and other ingredients, have been subsequently introduced (see p. 373.); sometimes, perhaps, by secretion during the cooling and consolidation of lavas.