192 GEOLOGY OP THE HIGH PLATEAUS. 



smaller than at present. A few large eruptions, however, reach out west- 

 ward, producing the sinuous course of the boundary which marks their 

 termination. One of these westerly projecting masses separates Bear Val- 

 ley into two portions, connected by a narrow gorge cut through it by 

 erosion. 



Overlooking Upper Bear Valley from the eastward stand two con- 

 spicuous mountain masses called Bear Peak and Little Creek Peak, of 

 which the respective elevations are 9,870 and 10,040 feet. Although 

 of moderate altitudes, they present, in consequence of their isolation, a 

 very commanding appearance and attract the attention from every point 

 of view in the surrounding country. They are also interesting on account 

 of their structure and the masses which constitute their bulk. The beds 

 which lie at their foundations merit some description. 



Wherever we examine the contact of the volcanics with the sediment- 

 ary beds along the western verge of the eruptive rocks of the Markagunt, 

 we usually find a series of strata composed of finely comminuted volcanic 

 materials. Sometimes it is a fine sandstone; sometimes an argillaceous 

 rock with minute fragments of feldspar and mica; sometimes a calcareous 

 or marly deposit. Often rolled and rounded fragments of notable size are 

 included, and the beds have then a coarse or gravelly texture, the grains 

 being fragments of some eruptive mass so much decomposed that it is dif- 

 ficult to determine its exact variety. These beds are always well stratified 

 and have clearly been deposited by water, and do not differ from ordinary 

 sedimentary beds, except in the fact that the materials which make up their 

 mass have been derived from eruptive rocks. The individual beds are 

 usually of small superficial extent and small thickness, and are often 

 seen running out with "feather-edges." They always overlie the system- 

 atic lacustrine Tertiaries of early Eocene age. Similar formations are 

 found at the northern and southern extremities of the Sevier Plateau and 

 in the East Fork Canon, where they have been more or less metamor- 

 phosed. They are exhibited on the west side of Bear Valley and again 

 along the base of the great trachytic wall of the Markagunt in considera- 

 ble variety. Wherever found they seem to constitute a group by them- 

 selves of more recent age than the uppermost Tertiaries of the Wasatch 



