268 GEOLOGY OF THE HIGH PLATEAUS. 



members, but are less numerous and of less thickness in the .aggregate. 

 Whether once continuous or not, it seems evident that the separation of 

 these two mountains from the plateau was effected by the gradual excavation 

 and enlargement of Summit Valley. As we view the objects on the ground 

 and try to reconcile ovirselves to this notion, the magnitude of the process 

 seems to make it incredible. Yet, as a common canon valley is the self- 

 evident result of erosion, so may such a valley as this be produced by the 

 oper-ation of the same general process, if sufficiently long continued. And 

 this valley is very ancient. It is a remnant of a topography existing before 

 the general ujilifting of the platform on which the plateau and mountains 

 stand. The volcanic rocks are probably as old as the Miocene, and the 

 inception of Summit Valley may have occurred late in that age or in the 

 early Pliocene. Judging comparatively by the effects of erosion here and 

 in the adjoining country, the isolation of such a mountain as Mount Mar- 

 vine is by no means a disproportionate work, when the duration of the 

 process is considered. This view is abundantly confirmed when we exam- 

 ine the positions of the Tertiary strata beneath the lavas. There has been 

 no downthrow sufficient to cause the valley, and the beds are seen to curve 

 gradually downwards towards the west in their normal attitudes on the 

 shoulder of the great monoclinal. (See Section 3, Atlas sheet. No. 6.) 



Mount Terrill is a long narrow ridge, consisting of trachytic lavas, rest- 

 ing upon calcareous beds of Lower Eocene age. The trachytes are rather 

 thin, their aggregate thickness being from 250 to 450 feet only. The 

 varieties are very similar to those of the Fish Lake Plateau. The extreme 

 summit is a remnant of a light-gray clinkstone (not phonolite, but a 

 sanidin-trachyte), which weathers into slabs about 3 inches thick by hori- 

 zontal planes of cleavage and by vertical joints. Underneath is a large 

 mass of light-red argilloid trachyte and several bodies of light-gray 

 trachyte, and one dark mass which may be an augitic variety. The sedi- 

 mentary beds upon which they lie are not well exposed. As is almost 

 always the case at such high altitudes (over 10,000 feet), they are covered 

 with soil and talus. No fossils were discovered, but their continuity has 

 been traced with strata of known age, and these are found in the ravine 



