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Show AGE OF THE GKANITOID AND POEPHYEITIO EOCKS. 95 two eruptions of the same epoch may differ from each other without calling for a distinction in their classification; but the textural differences which we now observe are due to the different conditions under which similar or sensibly identical magmas have solidified. The granites have solidified probably at great depths in the earth and under enormous statical pressure, while volcanic rocks have solidified at the surface. Porphyries, which usually occur in dikes or in intrusive masses, have solidified at intermediate horizons, though under conditions probably more nearly approaching those of volcanic than of granitoid rocks. The Palaeozoic and Archaean ages may have had their volcanic rocks, differing in no assignable respect from those of recent date, and upon a scale as grand and equally varied, but denudation has dissipated them. The granitoid rocks now exposed to our view have been brought to the light of day only by an enormous erosion, which has removed the thousands of feet of strata beneath which they received their present texture. This explanation is fortunately capable of a test by comparison with the facts presented by the rocks themselves, and though all the facts have not been collected and studied in this light, yet our knowledge of their general scope and bearing is considerable, and my belief is that they fairly sustain the theory. The granites and syenites are almost invariably found in localities where denudation has proceeded through a long series of epochs and has been vast in amount.* They are usually associated with metamorphic rocks which have been laid bare by the removal of great masses of superincumbent strata. They are not often found as interjected beds in unaltered or little altered Palaeozoic or Mesozoic strata; much less as contemporaneous flows. The eruptive syenites and granites, therefore, harmonize with the theory. The diorites and diabases have a different mode of occurrence. The diorites, so far as known, are believed to be almost invariably intrusive,! either in the form of dikes or intercalary between sedimentary beds. The same also appears to be true of those diabases which possess an unquestionable granitoid texture. There are, indeed, many rocks to which the name *It would be impracticable here to enter into a full discussion,of particular cases without protracting the discussion indefinitely. The statement will, I think, be generally admitted, t Jukes and Geike, Manual of Geology. |