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Show 358 GRANITES OF DIFFERENT AGES. [Ch.XXV. numerous observers, amongst the earliest of whom we may cite Von Buch, who discovered in Norway a mass. of granite overlying an ancient secondary limestone, containing orthocerata and other shells and zoophytes*. A considerable mass of granite in Sky is described by Dr. Macculloch as incumbent on limestone and shale, which are of the age of the English lias t· The limestone, which, at a greater distance from the granite, contains shells, exhibits no traces of them near the junction of the igneous rock, where it has been converted into a pure crystalline marble t. This granite of Sky was at first termed' Syenite,' by which name many geologists have denominated the more modern granites; but authors have entirely failed in their attempt to establish a distinction between granites and syenites on mineralogical characters. The latter have sometimes been defined to consist of a triple compound of felspar, quartz, and hornblende, but the oldest granites are very commonly composed of these ingredients only. In his later publications Dr. Macculloch has with great propriety, we think, called the plutonic rock of Sky a granite§. In different parts of the Alps a comparatively modern granite is seen penetrating through secondary strata, which contain belemnites, and other fossils, and are supP.osed to be referrible to the age of the English lias. According to the observations of M. Elie de Beaumont and Hugi, masses of this granite are sometimes found partially overlying the secondary beds, and altering them in a manner which we shall describe more particularly when we treat of the changes in composition and structure superinduced upon sedimentary deposits in contact with Plutonic rocksll (see wood-cut, No. 90, p. 371). In such examples we can merely affirm, that the granite is "' Travels through Norway and Lapland, P• 45. London, 1813. t See Murchison, Geol. Trans., 2nd Series, vol, ii. part ii. p. 311-321. t Western Islands, vol. i, p. 330, § Syst. of Geol., vol. i. p. 150. 11 Elie de Beaumont, Sur les Montagnes de l'Oisans, Mem. de Ia Soc. d'Hist. Nat. de Paris, tome v. Hugi, NMtur •. Historische Alpenreil!e, Soleure, 1830. Ch.XXV.] TRAP ROCKS, 350 newer than a secondary formation containing belemnites, but we can form no conjecture when it originated~ not even whether it be of secondary or tertiary date. It is, indeed, very necessary to be on our guard against the inference that a granite is usually of about the same age as the group of strata into which it has intruded itself, for in that case we shall be inclined to assume rashly that the granites found penetrating a more modern secondary rock, such as the lias for example, are much newer than those found invading strata older than the carboniferous series. The contrary may often be true, for the plutonic rock which was last in a melted state, may not have been forced up anywhere so near the surface as to enter into the newer groups of strata, and it may have been injected into a part of the earth's crust formed exclusively of the older sedimentary formations. 'In a deep series of strata,' says Dr. Macculloch, 'the superior or distant portions may have been but slightly disturbed, or have entirely escaped disturbance, by a granite which has not emitted its veins far beyond its immediate boundary. However certain, therefore, it may be~ that any mass of granite is posterior to the gneiss, the micaceous schist, or the argillaceous schists, which it traverses, or into which it intrudes, we are unable to prove that it is not also posterior to the secondary strata that lie above them *.' There can be no doubt, however, that some granites are more ancient than any of our regular series which we identify by organic remains, because there are rounded pebbles of granite, as well as gneiss, in the conglomerates of the oldest fossiliferous groups. Distinction between ·volcanic and plutonic rocks-Trap.The next point to consider is the distinction between the plutonic and volcanic rocks. When geologists first began to examine attentively the structure of the northern parts of Europe, they were almost entirely ignorant of the phenomena of existing volcanos, and when they met with basalt and other "' Syst. of Geol., vol, i. p.:l36. |