The Student's Elements of Geology Part 66

(FIGURE 594. Trap dividing and covering sandstone near Suishnish, in Skye.

(MacCulloch.))

In the Hebrides and other countries, the same ma.s.ses of trap which occupy the surface of the country far and wide, concealing the subjacent stratified rocks, are seen also in the sea-cliffs, prolonged downward in veins or dikes, which probably unite with other ma.s.ses of igneous rock at a greater depth. The largest of the dikes represented in Figure 594, and which are seen in part of the coast of Skye, is no less than 100 feet in width.

Every variety of trap-rock is sometimes found in dikes, as basalt, greenstone, feldspar-porphyry, and trachyte. The amygdaloidal traps also occur, though more rarely, and even tuff and breccia, for the materials of these last may be washed down into open fissures at the bottom of the sea, or during eruption on the land may be showered into them from the air. Some dikes of trap may be followed for leagues uninterruptedly in nearly a straight direction, as in the north of England, showing that the fissures which they fill must have been of extraordinary length.

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 heat of melted matter and of the entangled steam and gases might be expected to cause.

PLAS-NEWYDD: DIKE CUTTING THROUGH SHALE.

A striking example, near Plas-Newydd, in Anglesea, has been described by Professor Henslow. (Cambridge Transactions volume 1 page 402.) The dike is 134 feet wide, and consists of a rock which is a compound of feldspar 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, of 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 porcelanous 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. (Ibid.

volume 1 page 410.) 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. (Ibid. volume 2 page 175.)

ANTRIM: DIKE CUTTING THROUGH CHALK.

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 eight or ten 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 porcelanous 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." (Dr. Berger Geological Transactions 1st series volume 3 page 172.) All traces of organic remains are effaced in that part of the limestone which is most crystalline.

(FIGURE 595. Basaltic dikes in chalk in Island of Rathlin, Antrim. Ground-plan as seen on the beach. (Conybeare and Buckland. (Geological Transactions 1st series volume 3 page 210 and plate 10.

From left to right: chalk: dike 35 ft.: dike 1 ft.: dike 20 ft.: chalk.)

Figure 595 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. By another, the shale of the coal-measures has been indurated, a.s.suming the character of flinty slate; and in another place the slate-clay of the lias has been changed into flinty slate, which still retains numerous impressions of ammonites. (Ibid.

volume 3 page 213; and Playfair Ill.u.s.tration of Huttonian Theory s. 253.)

It might have been antic.i.p.ated that beds of coal would, from their combustible nature, be affected 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 nine feet on each side. At c.o.c.kfield Fell, in the north of England, a similar change is observed. Specimens taken at the distance of about thirty 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. (Sedgwick Cambridge Transactions volume 2 page 37.)

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 to 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 being 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 heat supplied by the dike and its gases will act more powerfully.

INTRUSION OF TRAP BETWEEN STRATA.

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 were caused by gases propelling the lava upward.

RELATION OF TRAPPEAN ROCKS TO THE PRODUCTS OF ACTIVE VOLCANOES.

When we reflect on the changes above described in the strata near their contact with trap dikes, and consider how complete is the a.n.a.logy or often ident.i.ty in composition and structure of the rocks called trappean and the lavas of active volcanoes, 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. A large portion of the trappean rocks first studied in the north of Germany, and in Norway, France, Scotland, and other countries, were such as had been formed entirely under 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 the 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 downward 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.

(FIGURE 596. Strata intercepted 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 Chapter 6); and this fact prepares us to expect a similar destruction of whatever may once have formed the uppermost part of ancient submarine or subaerial volcanoes, 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 Figure 596), 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 volcanoes.

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 above); sometimes, perhaps, by secretion during the cooling and consolidation of lavas. In the Little c.u.mbray, one of the Western Islands, near Arran, the amygdaloid sometimes contains elongated cavities filled with brown spar; and when the nodules have been washed out, the interior of the cavities is glazed with the vitreous varnish so characteristic of the pores of slaggy lavas. Even in some parts of this rock which are excluded from air and water, the cells are empty, and seem to have always remained in this state, and are therefore undistinguishable from some modern lavas. (MacCulloch Western Islands volume 2 page 487.)

Dr. MacCulloch, after examining with great attention these and the other igneous rocks of Scotland, observes, "that it is a mere dispute about terms, to refuse to the ancient eruptions of trap the name of submarine volcanoes; for they are such in every essential point, although they no longer eject fire and smoke."

The same author also considers it not improbable that some of the volcanic rocks of the same country may have been poured out in the open air. (System of Geology volume 2 page 114.)

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 volcanoes, 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.

CHAPTER XXIX.

ON THE AGES OF VOLCANIC ROCKS.

Tests of relative Age of Volcanic Rocks.

Why ancient and modern Rocks can not be identical.

Tests by Superposition and intrusion.

Test by Alteration of Rocks in Contact.

Test by Organic Remains.

Test of Age by Mineral Character.

Test by Included Fragments.

Recent and Post-pliocene volcanic Rocks.

Vesuvius, Auvergne, Puy de Come, and Puy de Pariou.

Newer Pliocene volcanic Rocks.

Cyclopean Isles, Etna, Dikes of Palagonia, Madeira.

Older Pliocene volcanic Rocks.

Italy.

Pliocene Volcanoes of the Eifel.

Tra.s.s.

Having in the former part of this work referred the sedimentary strata to a long succession of geological periods, we have now to consider how far the volcanic formations can be cla.s.sed in a similar chronological order. The tests of relative age in this cla.s.s of rocks are four: first, superposition and intrusion, with or without alteration of the rocks in contact; second, organic remains; third, mineral characters; fourth, included fragments of older rocks.

Besides these four tests it may be said, in a general way, that volcanic rocks of Primary or Palaeozoic antiquity differ from those of the Secondary or Mesozoic age, and these again from the Tertiary and Recent. Not, perhaps, that they differed originally in a greater degree than the modern volcanic rocks of one region, such as that of the Andes, differ from those of another, such as Iceland, but because all rocks permeated by water, especially if its temperature be high, are liable to undergo a slow trans.m.u.tation, even when they do not a.s.sume a new crystalline form like that of the hypogene rocks.

Although subaerial and submarine denudation, as before stated, remove, in the course of ages, large portions of the upper or more superficial products of volcanoes, yet these are sometimes preserved by subsidence, becoming covered by the sea or by superimposed marine deposits. In this way they may be protected for ages from the waves of the sea, or the destroying action of rivers, while, at the same time, they may not sink so deep as to be exposed to that Plutonic action (to be spoken of in Chapter 31) which would convert them into crystalline rocks. But even in this case they will not remain unaltered, because they will be percolated by water often of high temperature, and charged with carbonate of lime, silex, iron, and other mineral ingredients, whereby gradual changes in the const.i.tution of the rocks may be superinduced. Every geologist is aware how often silicified trees occur in volcanic tuffs, the perfect preservation of their internal structure showing that they have not decayed before the petrifying material was supplied.

The porous and vesicular nature of a large part, both of the basaltic and trachytic lavas, affords cavities in which silex and carbonate of lime are readily deposited. Minerals of the zeolite family, the composition of which has already been alluded to in Chapter 28, occur in amygdaloids and other trap-rocks in great abundance, and Daubree's observations have proved that they are not always simple deposits of substances held in solution by the percolating waters, being occasionally products of the chemical action of that water on the rock through which they are filtered, and portions of which are decomposed. From these considerations it follows that the perfect ident.i.ty of very ancient and very modern volcanic formations is scarcely possible.

TESTS BY SUPERPOSITION.

(FIGURE 597. Section through sedimentary ma.s.s with melted matter.)

If a volcanic rock rest upon an aqueous deposit, the volcanic must be the newest of the two; but the like rule does not hold good where the aqueous formation rests upon the volcanic, for melted matter, rising from below, may penetrate a sedimentary ma.s.s without reaching the surface, or may be forced in conformably between two strata, as b below D in Figure 597, after which it may cool down and consolidate. Superposition, therefore, is not of the same value as a test of age in the unstratified volcanic rocks as in fossiliferous formations. We can only rely implicitly on this test where the volcanic rocks are contemporaneous, not where they are intrusive. Now, they are said to be contemporaneous if produced by volcanic action which was going on simultaneously with the deposition of the strata with which they are a.s.sociated. Thus in the section at D (Figure 597), we may perhaps ascertain that the trap b flowed over the fossiliferous bed c, and that, after its consolidation, a was deposited upon it, a and c both belonging to the same geological period. But, on the other hand, we must conclude the trap to be intrusive, if the stratum a be altered by b at the point of contact, or if, in pursuing b for some distance, we find at length that it cuts through the stratum a, and then overlies it as at E.

(FIGURE 598. Section through sedimentary ma.s.s with melted matter.)

We may, however, be easily deceived in supposing the volcanic rock to be intrusive, when in reality it is contemporaneous; for a sheet of lava, as it spreads over the bottom of the sea, can not rest everywhere upon the same stratum, either because these have been denuded, or because, if newly thrown down, they thin out in certain places, thus allowing the lava to cross their edges. Besides, the heavy igneous fluid will often, as it moves along, cut a channel into beds of soft mud and sand. Suppose the submarine lava F (Figure 598) to have come in contact in this manner with the strata a, b, c, and that after its consolidation the strata d, e are thrown down in a nearly horizontal position, yet so as to lie unconformably to F, the appearance of subsequent intrusion will here be complete, although the trap is in fact contemporaneous.

We must not, therefore, hastily infer that the rock F is intrusive, unless we find the overlying strata, d, e, to have been altered at their junction, as if by heat.

The test of age by superposition is strictly applicable to all stratified volcanic tuffs, according to the rules already explained in the case of sedimentary deposits (see Chapter 8).