Xviii GEOLOGY OP 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 dis- 

 tricts 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 {jroceeded 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 cor- 

 respond 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 vol- 

 canic piles, and so vast is the mass which has been torij down and scattered, 

 that the work of restoration is exceedingly difficult. The task of finding 



