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Show 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 ourselves 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 operation 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 uplifting 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 examine 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, resting 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 horizontal 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 sedimentary 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 / |
