Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/322

Rh 308 (sometimes glassy or felsitic) st.ruct.ure have been in- jected in a ﬂuid or at least viscous condition into other rocks, or having been blown into fragments have consoli- dated iu volcanic pipes. After some practice in the ﬁeld the geologist. learns to recognize these rocks, and to distin- guish them from the similar masses which must be placed in the contemporaneous series. As a rule their crystalline texture is coarser than in that series; only in a few rare cases does a cellular or amygdaloidal character appear, and the fragmental accompaniments so characteristic of the con- temporaneous sheet are only found in the actual vents of eruption. Granite, syenite, felsite, diorite, basalt, and agglomerate occur in this form. The general law which has governed the intrusion of igneous rock within the earth’s crust may be thus stated : every ﬂuid mass impelled upwards by pressure from below, or by the expansion of its own imprisoned vapour, has sought egress along the line of least resistance. What that line was to be has depended in each case upon the structure of the terrestrial crust and the energy of eruption. In many instances it has been determined by an already existent dislocation; in others by the planes of stratiﬁcation, or by the surface of junction of two unconformable forma- tions, or by irregular cracks and rents, or by other more complex lines of weakness. Sometimes the intruded mass has actually fused and oblit.erated some of the rock which it has invaded, incorporating this portion into its own substance. The shape of the channel of escape has neces- sarily determined the form of the intrusive rock, as the mould regulates the form assumed by a mass of cast-iron. This offers a. very convenient means of classifying the intrusive rocks. According to the shape of the mould in which they have solidiﬁed, they may be arranged as—(l) amorphous masses, sheets, (3) veins and dykes, and (4) necks. _I. AMORPHOUS .LA.ssEs.—These consist chieﬂy of crys- talline coarse-textured rocks. Granite and syenite are the most conspicuous, but there are to be included also various quartz—porphyries, felsites, diorites, &c. Where rocks occur in this form which also are found in sheets and dykes as well as contemporaneous beds, it is com- monly observed that they are more coarsely crystalline in the form of amorphous masses than in any other. Doleritic rocks afford many examples of this characteristic. G'ram'tz'c Bosses.—It was once a ﬁrmly-held tenet that granite is the oldest of rocks, the foundation on which all other rocks have been laid d.own. This idea no doubt originated in the fact that granite is found rising from beneath gneiss, schist, and other crystalline masses which in their turn underlie very old stratiﬁed formations. The intrusive character of granite, shown by its numerous rami- fying veins, proved it to be later than at least those rocks which it had invaded. Nevertheless the composition and structure of gneiss and mica-schist were believed to be best explained by supposing these rocks to have been derived from the waste of granite, and thus, though the existing intrusive granite had to be recognized as posterior in date, it was regarded as only a subsequent protrusion of the vast underlying granitic crust. In this way the idea of the primeval or fundamental nature of granite held its ground. . From what has already (ante, p. 258) been said regard- mg the fusion and consolidation of rocks, and the evidence supplied on this subject by granite itself, it will readily be understood that the first or original crust could hardly have been one of granite. That rock, so far as can be made out by careful microscopic examination, appears to have always consolidated under considerable pressure, and in the pre- sence of superheated water and even of liquid carbonic GEOLOGY by igneous rocks which, either possessing a crystalline’ [1v. acid——-conditions which probably never obtained at the earth's immediate surface. The original crust may have been of a glassy character like some of the vitreous lavas ; but whatever it was, no trace of it has ever been or is ever likely to be found. The presence of granite at the existing surface m11st in all cases be due to the removal by denudation of the masses of rock under which it originally consolidated. The fact that, wherever extensive denudation of an ancient series of crystalline rocks has taken place, a subjacent granite nucleus is apt to appear does not prove that rock to be of a prim- eval origin. It shows, however, that the lower portions of crystalline rocks very generally assume a granitic type, and it suggests that if at any part of the earth we could bore deep enough into the crust we should probably come to a granitic layer. That this layer, even if general round the globe, is not always of the highest geological antiquity is abundantly clear from the fact that in many cases it can be proved to be of lat.er (late than fossiliferous formations the geological position of which is known ; that is, the granitic layer has invaded these formations, rising up through them, and probably melting down portions of them in its progress. This is true not only of ancient. Palaeozoie but of other stratiﬁed rocks of various nmch more recent ages. So that we must conclude that granite does not belong exclusively to the earliest nor to any one geo- logical period, but rather that it has been formed at various epochs, and may even be forming now, wherever the con- ditions required for its production have existed. As a matter of fact granite occurs much more frequently in association with older and therefore lower than with newer and higher rocks. But a little retlexion shows us that this must be the case. Granite having a deep-seated origin must rise through the lower and more ancient masses before it can reach the overlying more recent formations. But many protrusions of granite would doubtless never ascend beyond the lower rocks. Subsequent denudation would be needed to reveal these protrusions, and this very process would remove the later formations and at the same time any portions of the granite which might have reached them. Granite frequently occurs in the central parts of 11101111- tain chains ; sometimes it forms there a kind of core round which the various gneisses, schists, and other crystalline rocks are arranged with more or less irregularity. Mom. frequently it appears in large eruptive bosses which traverse indifferently the rocks on the line of which they rise. Sometimes it even overlies the schistose and other rocks, as in the Piz (le Graves in the upper Engadine, where a wall-like mass of granite, with syenit.e, diorite, and altered rocks, may be seen resting upon schists. In the Alps and other mountain ranges it is found likewise in large bed-like masses which run in the same general direction as the rocks with which they are associated. Many of the most characteristic features of granitic bosses can be admirably studied where the rock has risen through contorted sedimentary formations, which form undulating or hilly ground rather than mountains. The granite of the south-east and east of Ireland, the south of Scotland, and the south-west of England may be taken as illustrative examples. _ In the south-east of Ireland a mass of granite 70 miles in length and from 7 to 17 in width stretches from north- east to south-west, nearly along the strike of the Lower Silurian rocks. These strata, however, have not been upraised by it in such a way as to expose their lowest beds dipping away from the granite. On the contrary, they seem to have been contorted prior to the appearance of that rock ; at least they often dip towards it, or lie horizontally or undulate upon it, apparently without any reference to STI-KUCTUR.- L.