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Show METAMORPHISM OF FRAGMENTAL VOLCANIC ROCKS. 79 around them and the fragments contained in the latter agree with the rocks remaining in the former. But there is much complication and obscurity in many instances arising from the fact that these eruptive centers have again and again been active, the work of one epoch being overflowed and partially masked by the extravasation and still later devastation of subsequent epochs. Moreover, the loftiest points are composed of massive rocks, and the positions of the conglomerates are invariably below those of the centers from which they are presumed to have emanated, except in those cases where the relative altitudes have been changed by relatively recent displacement. The general problem would have been full of anomalies, however, were we not in a position to unravel both the complications arising from vertical movements and those from the recurrence of the volcanic activity. But being able to restore in imagination the displaced blocks of country, and in a considerable measure to separate into periods the course of volcanic activity, we find by so doing that the difficulties vanish and the facts group themselves into normal relations. A very striking characteristic of these clastic volcanic rocks, both the tufas and the conglomerates, is their great susceptibility to metamorphism. Not only have the beds in many localities been thoroughly consolidated, but they have undergone crystallization. Those tufas and conglomerates which are of older date, and which have been buried beneath more recent accumulations to considerable depths, rarely fail to show conspicuous traces of alteration, and in many cases have been so profoundly modified, that for a considerable time there was doubt as to their true character. The general tendency of this process is to convert the fragmental strata into rocks having a petrographic facies and texture very closely resembling certain groups of igneous rocks. When we examine the beds in situ no doubt can exist for a moment that they are waterlaid strata. (See heliotypes V and VI.) The hand specimens taken from beds which are extremely metamorphosed might readily pass, even upon close inspection, for pieces of massive eruptive rocks, were it not that the original fragments are still distinguishable, partly by slight differences of color, partly by slight differences in the degree of coarseness of texture. But the matrix has become very similar to the included fragments, holding the same kinds of crystals, |