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Show 96 GEOLOGY OF THE HIGH PLATEAUS. of diabase is given by some lithologists, but which-are really dolerites and basalts, bearing indications of a volcanic origin, and these are found as contemporary or interbedded coulees. They differ notably, however, from the intrusive diabases, though they are sometimes confounded with them. In short, the ancient eruptives which remain as coulees have the volcanic textures, and those which remain as intrusives have the granitic or sometimes the porphyritic texture, and the diorites and diabases equally with the syenites and granites present no obstacle to Von Cotta's hypothesis, but are to all appearances in full accord with it. It is as certain as anything in geological science can well be that the texture of the granitoid eruptive rocks could not have been derived (at least directly) from any special conditions existing prior to their eruption. Every theory must presuppose that during their eruption or intrusion they were plastic, and that a portion of their groundmass, if not the whole of it, was amorphous and in a condition of igneous or aqueo-igneous fusion, and in such a condition it is little less than absurd to suppose that any texture at all resembling granite could have prevailed. The closely interlocked crystals of such a groundmass are as antithetical to the very idea of plasticity as it is possible to conceive. The crystalline texture must surely have been a development altogether subsequent to plastic movement.* There is, therefore, a lurking fallacy in the statement that granitoid rocks had their periods of eruption in the earlier ages, while the volcanics had theirs in Tertiary time. The true and rational mode of stating the case maybe this: that through all the ages igneous magmas have been erupted, which have, according to their final resting-places and the conditions there existing, consolidated either into granitoid or half-crystalline rocks. The magmas themselves have been the same in all ages, each to each within its own group, and so too have the resulting rocks each to each under equivalent conditions of consolidation. We find in the Tertiaries only volcanic rocks, because the corresponding granitoids are far beneath them and not yet laid bare by secular erosion. We find among Pre-Tertiary eruptions chiefly granitoids, because the corresponding volcanics have been swept away. * It is of course intelligible that some crystals may have existed in an amorphous fluent paste during the eruption. |