Part 60 (2/2)
[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.
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