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Show 362 GRANITE AND TRAP. [Ch. XXV. nating the volcanic from the plutonic rocks is sufficiently great; for we must draw an arbitrary line between them, there being an insensible passage from the most common forms of granite into trap or lav~. . , ' The ordinary gramte of Aberdeenshue, says Dr. Mac-culloch, 'is the usual ternary compound of quartz, felspar, and mica, but sometimes hornblende is added to these, or the hornblende is substituted for the mica. But in many places a variety occurs which is composed si~ply of ~elspar . and hornblende, and in examining more mmutely thts duphcate compound, it is observed in some places to assume a fine grain, and at length to become undistinguishable from the greenstones of the trap family. It also passes in the same uninterrupted manner into a basalt, and at length into a soft claystone, with a schistose tendency on exposure, in no respect differing from those of the trap islands of the western coast*.' The same author mentions, that in Shetland a granite composed of hornblende, mica, felspar, and quartz, graduates in an equally perfect manner into basalt t. It would be easy to multiply examples to prove that the granitic and trap-rocks pass into each other, and are met:ely different forms which the same elements have assumedaccordmg to the different circumstances under which they have consolidated from a state of fusion. What we have said respecting the mode of explaining the different texture of the central and external parts of the Vesuvian dikes may enable the reader in some measure to comprehend how such differences may originate t· The same lava which is porous where it has flowed over from the . crater, and where it has cooled rapidly and under comparatively slight pressure, is compact and porphyritic in the dike. Now these dikes are evidently the channels of com· munication between the crater and the volcanic foci below; so that we may suppose them to be continuous to the depth of several hundred fathoms, or perhaps two or three miles, or eve~ more; and the fluid matter below, which cools and consoh- • Syst. of Geol., vol. i. p. 157. t Ibid., p. 158. t See above, P• 124. Ch.XXV.] ORIGIN OF GRANITE, 363 dates slowly under so enormous a pressure, may be supposed to acquire a very distinct texture and become granite. If it be objected that we do not find in mountain-chains volcanic dikes passing upwards into lava, and downwards into granite, we may answer that our vertical sections are usually of small extent, and it is enough that we find in certain localities a transition from trap to porous lava, and in others a passage from granite to trap. It should also be remembered, that a large proportion of the igneous rocks, when first formed cannot be supposed to reach the surface, and these may assume' the usual granitic texture without graduating into trap, or into such lava and scorire as are found on the flanks of a volcanic cone. Theory of the origin of granite at all periods.-It is not uncommon for lava-streams to require more than ten years to cool in the open air, and a much longer period where they are of great depth. '!"'he melted matter poured out from Jorullo, in Mexico, in the year 1759, which accumulated in some places to the height of 550 feet, was found to retain a high temperature half a century after the eruption*· For what immense periods, then, must we not conclude that great masses of subterranean lava in the volcanic foci may remain in a red hot or incandescent state, and how gradual must be the process of refrigeration! This process may be sometimes retarded for an indefinite period, by the accession of fresh supplies of heat, for we find that the lava in the crater of Stromboli, one of the Lipari islands, has been in a state of constant ebullition for the last 2000 years, and we must suppose this fluid mass to communicate with some cauldron or reservoir of fused matter below. In the Isle of Bourbon, also, where there has. been an emi~sion of lava once in every two years for a long penod, we may mfer that the lava below is permanently in a state of liquefaction. When melted matter is injected into the fissures of a contiguous rock at a considerable depth, it may cool rapidly if that rock has not acquired a high temperature; but suppose, on he ~ See vol. i. p. 378, and Second Edition, p. 433. |
