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Show xviii GEOLOGY OF THE HIGH PLATEAUS. Rocks," this subject has been of peculiar interest to American students of western geology. The discussion of it as applied to the District of the High Plateaus* will be found in the third chapter. The great conglomerates composed of fragmental volcanic materials also furnished an interesting subject of inquiry. There are many other districts in the West where similar masses are found sometimes in even greater quantity, and their origin and mode of accumulation became an attractive problem. That these formations are accumulations of ejected fragments seemed inadmissible, and the further the investigation proceeded the more untenable did this view appear to be. While great bodies of tufaceous matter are usually found surrounding volcanic orifices, the conglomerates in question do not conform either in the structure of the beds or in the dis tribution of their masses to those of ordinary tufa cones. At the present time there are now accumulating in the valleys between the great tables extensive alluvial formations, which upon careful examination seem to correspond closely to the older conglomerates now exposed in the palisades of the plateaus, and the conclusion was reached that the ancient conglomerates and modern alluvia were produced by the same process. The discussion of these formations is contained in the tenth chapter, and the conclusions are embodied in the latter part of the third chapter. Another interesting subject was the metamorphism of clastic beds derived from the detritus of volcanic rocks, and it is treated in the latter part of the eleventh chapter relating to the East Fork Canon in the Sevier Plateau. Very naturally one of the most prominent objects of investigation was to find the localities in which were situated the vents or orifices from which the great eruptive masses were outpoured. In the case of the basalts, which are comparatively recent in their dates of eruption, there was in most. cases no difficulty. But with the older rocks, the rhyolites, trachytes, and andesites, it is quite different. Some of the rhyolites show very plainly even to the most superficial investigation whence they came. Others do not. So powerfully have the destroying agents wrought upon the old volcanic piles, and so vast is the mass which has been torn down and scattered, that the work of restoration is exceedingly difficult. The task of finding |
