ALLUVIAL FORMATIONS— THOUSAND LAKE MOUNTAIN. 279 



These alluvial masses are partly conglomeritic in textvrre, especially 

 near the borders of the lava sheets and at the foot of the plateau slopes. 

 Towards the middle of the valley they become finer, shading into sandy or 

 fine gravelly deposits. They are instances of the formation of conglom- 

 erates upon a considerable scale by the alluvial process, but under condi- 

 tions somewhat different from those disclosed in Sevier and Grass Valleys. 

 The included fragments upon the western and southern portions are always 

 volcanic and exceedingly varied. The debris derived from Thousand Lake 

 Mountain on the eastern side consists mainly of fine quartz sand coming 

 from the decay of the Jurassic and Triassic sandstones of that structure. 



In the northwestern part of Rabbit Valley a few exposures of Ter- 

 tiary beds are found beneath the terminal trachytic sheets. Upon the 

 eastern side of the valley are still better exposures upon the lowest slopes 

 of Thousand Lake Mountain. In the latter locality they are soon cut off 

 by the great fault, and reappear nearly 4,000 feet above, beneath the lava 

 cap upon the summit. Below they abut against the Lower Trias. Laterally 

 they run beneath extensive outpours of basalt, which, though not of very 

 modern origin, are still comparatively recent. 



THOUSAND LAKE MOUNTAIN. 



Thousand Lake Mountain is an exceedingly interesting object. The 

 name was given by the Mormons who pasture flocks in the valley below. 

 They derived it from a group of pools of glacial origin upon the sum- 

 mit. Structurally and morphologically it is a small plateau, in some 

 respects very similar to the other and larger members of the district, but 

 possessing, also, features peculiar to itself. The country to the west of it is 

 thrown down by a profound fault forming the depression of Rabbit Valley, 

 The country to the east of it is the inner region of the Plateau Province, 

 from which thousands of feet of strata have been removed by the grand 

 erosion of Tertiary time, while the mountain itself has been left like a 

 gigantic butte or cameo upon the border of the region. Upon its southern 

 flank the Fremont River has cut a wide passage, which has separated it 

 from its mighty parent, the Aquarius Plateau. 



Upon the summit is a lava cap from 400 to 500 feet in thickness and 



