SUMMIT VALLEY. 259 



about 9,300 feet, brings us upon the floor of Summit Valley. Upon the west 

 is a sharp crest-line, constituting the eastern verge of Fish Lake Plateau, 

 which overlooks the valley from an altitude of 11,000 to 11,400 feet. 

 Upon the east side rise two conspicuous masses — Mount Terrill and Mount 

 Marvine. This valley is an excellent starting-point, from which .we may 

 make excursions radiating in many directions, and study in detail the diver- 

 sifted objects which compose the surrounding country. And, first, let us 

 look at the nature of the valley itself. 



Not the smallest among its attractions for the geologist is the fact that 

 it is a most eligible summer camping-place. In the daytime, through- 

 out. July, August, and most of September, it is mild and genial, while 

 the nights are frosty and conducive to rest. The grass is long, luxuriant, 

 and aglow with flowers. Clumps of spruce and aspen furnish shade 

 from the keen rays of the sun, and fuel is in abundance for camp-fires. 

 Thus the great requsites for Western camp-life, fuel, water, and grass, are 

 richly supplied, while neither is in such excess as to be an obstacle to pro- 

 gress and examination. 



The valley floor is, for the most part, Lower Tertiary. For a consider- 

 able portion of the length the edges of these beds are exposed upon the 

 eastern side of the valley, forming the lower slopes of Mounts Terrill and 

 Marvine. They are also seen at the base of the Fish Lake slopes; but a 

 little higher up they are covered with ancient lavas. Northward, however, 

 lavas form the floor of the valley. Proceeding in that direction a few miles, 

 the mountain-walls which inclose the valley rapidly decline in altitude 

 and die away in steep slopes, while the platform on which we travel at 

 length becomes the summit of a plateau, having an altitude about 2,000 

 feet lower than the neighboring tables; and projecting 4 or 5 miles farther 

 northward, it ends in abrupt volcanic cliffs, from the crests of which we 

 overlook all the space which intervenes between them and the Wasatch 

 Plateau, 20 miles distant. The thickness of the lava at these cliffs is about 

 700 feet, and is composed of hornblendic trachytes in very massive sheets, 

 alternating with augitic andesites, which are much thinner. Retracing our 

 steps and traveling to the southern end of the valley, we find its floor undu- 

 lating with little hills, a part of which are Eocene beds and a part are old 



