Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/323

Rh PLUTONIC IGNEOUS ROCKS .] movements which it could have produced. As Mr Jukes has shown, the Silurian strata are underlaid bya vast mass of Cambrian rocks, all of which must have been invaded by the granite before it could have reached its present horizon. He infers that the granite must l1ave slowly and irregularly eaten its way upward through the Silurian rocks, absorbing much of them into its own mass as it rose. For a mile or more the stratified beds next the granite have been altered into miea-schist, and are pierced by numerous veins from the invading rock. Within the margin of the granitic mass belts or rounded irregular patches of schist are enclosed ; but in the central tracts where the granite is widest, and where therefore we may suppose the deepest parts of the mass have been laid bare, no such included patches of altered rock oceur. From the manner in which the schistose belt is disposed round the granite, it is evident that the upper surface of the latter rock where it extends beneath the schists must be very uneven. Doubtless it rises in some places much nearer to the present surface of the ground than at otiiers, and sends out veins and strings which do not appear above ground. If, as Mr J ukes sup- poses, a thousand feet of the schists could be restored at some parts of the granite belt, no doubt the belt would there be entirely buried ; or if, on the 0tl1er hand, the same thickness of rock could be stripped off some parts of the band of schist, the solid granite underneath would be laid bare. The extent of granite surface exposed must thus be largely determined by the amount of denudation, and by the angle at which the upper surface of the granite is inclined beneath the schists. Where the inclination is high, prolonged denudation will evidently do comparatively little in widening the belt. But where the slope is gentle, and especially where the surface undulates, the removal for some distance of a con1paratively slight thickness of rock ma.y uncover a large breadth of underlying granite.‘ llecent observations by Professor Hull and Mr Traill of the (‘eological Survey have shown that in the Mourne lfountains a mass of granite has in some parts risen up through highly inclined Silurian rocks, which consequently seem to be standing almost upright upon an underlying boss of granite. The strata are sharpljf iruncated by the crystalline mass, and are indurated but not otherwise altered. The intrusive nature of the granite is well shown by the way i11 which numerous dykes of dark melaphyrc are cut off when they reach that rocls.2 In the Lower Silurian tract of the south of Scotland several large intrusive bosses of granite occur. The strata do 11ot dip away from them on all sides, but with triﬂing exceptions maintain their normal N. E. and SW. strike up to the granite on one side, and resume it again on the other. The granite indeed occupies the place of so 1nuch Silurian greywacke and shale. There is usually a metamor- phosed belt of variable width in which, as they approach the granite, the stratiﬁed rocks assume a schistose or gneissoid character. Numerous small, dark, often angular patches or fragments of mica-schist may be observed along the marginal parts of the granite. Similar features are presented by the granite bosses of Devon a11d Cornwall which have risen through Devonian str.1t-.1. The manner in which some bosses of granite penetrate the rocks among which they occur strongly reminds one of the structure of volcanic necks or pipes. The granite is found as a circular or elliptical mass which seems to descend vertically through the surrounding rocks without . seriously altering or disturbing them, as if a tube-shaped opening had been blown out of the crust of the earth up V.'lllCll the granite had risen. Several of the granite masses of the south of Scotland exhibit this character very strik-
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Ilorzzontal Section No. 22. Geol. Surv. Ireland. GEO L O G Y 30$) ingly. That granite and granitoid rocks have actually been associated with volcanic action is shown by the way in which they occur in connexion with the Tertiary volcanic rocks of Skye, Mull, and other islands in the Inner Hebrides. As M r J ukes suggested many years ago, granite or granitoid masses may lie at the roots of volcanoes, and may be the source whence the more silicated lavas, such as trachyte and liparite, proceed.3 That some granite, however, is of metamorphic origin, that is to say, l1as been produced by the gradual softening and recrystallization of other rocks at some depth within the crust of the earth, seems to be now satisfactorily estab- lished. Such granite may be looked upon as the extreme of metamorphism, the various schists and gneisses being less advanced stages of the process. Provided the chemical composition of the altered rock be the same as that of granite, it is not necessary that the granite resulting from its alteration should be supposed to differ in any noteworthy particular from ordinary intrusive or igneous granite. The members of the Geological Survey of Ireland have indeed distinguished two granites in Galway, one of which they regard as metamorphic, the other as igneous. The former is characterized by the occurrence of two felspars (orthoclase and oligoclase) ; the latter contains only one (orthoclase). More recently, however, in the east of the country they l1ave separated two groups of granites, of which the intru- sive masses are composed of dark-coloured quartz, ortho- clase, albite, and black mica (Mourne Mountains), while the metamorphic variety is formed of grey fclspar, quartz, and black mica. The mineralogical composition of granite formed by the metamorphism of other and specially sedi- mentary rocks must necessarily vary with that of the masses out of which it has arisen. In some cases there is a regular gradation from true granite outward into the schistose and gneissose masses. But this passage need not always occur, for if the granite was subject to unequal pressure (which it assuredly would in most cases be) it would in its soft, pasty condition undoubtedly be squeezed into any rents made in the surrounding rocks, and would thus imitate exactly a truly igneous mass, which in actual fact it would then be. When a mass of granite rises through unaltered or only locally altered strata, it may fairly be assumed to be igneous and intrusive. Vhen, on the other hand, it is intimately associated with extensive masses of schist and gneiss, many of which can only be distinguished from it by their foliated structure, its metamorphic origin may at least be strongly suspected. Fundamentally, indeed, igneous and metamorphic granite seem to be due only to different modifications of the same subterranean processes. A mass of originally sedimentary rocks may be depressed to a depth of several thousand feet within the earth’s crust, subjected there to vast pressure and considerable heat in presence of interstitial water or steam, and may thus be metamorphosed into crystalline schists. A portion of this mass, undergoing extreme alteration, may so completely lose all trace of its original ﬁssile structure as to become amorphous crystalline granite, some of which may even be thrust as veins into the less highly changed parts above and around. One stage further would bring before us a con- nexion opened between the surface and such a decp—seated granitic mass, and the consequent ascent and outburst of acid lavas and their fragmental accompaniments. Anzorplums Jfusscs of I)£o2'ite, (f'c.——On a smaller scale usually than granite, other crystalline rocks assume the condition of amorphous bosses. Sycnite, diorite, quartz- porphyry, and members of the basalt family have often been erupted in irregular masses, partly along ﬁssures, partly along the bedding, b11t often involving and appar- 3 Jlrlnual of (}'¢'0Io_r/3/, 2241 ed., p. 93; Geikie, Trans. Geol. Soc. E¢lz'u., ii. 301; Judd, Qzmrt. Joztrn. Gaol. Soc., xxx. 220.