660 DESCRIPTIVE GEOLOGY. 



SECTION III. 



FISH OKEEK AND BATTLE MOUNTAINS. 



BY ARNOLD HAGUE. 



Fish Ckeek Mountains. — The Fish Creek Mountains lie directly north 

 of, and in the same trend with, the Augusta Mountains, and, like the latter, 

 are made up mainly of rhyolitic outbursts. From the nature of the volcanic 

 material, they form an irregular group of hills, a roughly diamond-shaped 

 mass, the longer axis stretching northeast and southwest for 25 miles, while 

 across their broadest expanse they measure about 1 8 miles. The more 

 elevated portion is situated in the extreme southernmost end, culminating 

 in Mount Moses, which rises in a conical peak, 8,725 feet above sea-level, 

 and falls away in sharp ridges with steep slopes toward the west, south, and 

 east. Northward the country passes into a broad undulating table, inclined 

 gently to the southeast, with deeply-eroded canons along its edges, pene- 

 trating for long distances into the main mass. On the north side, this table 

 shows a somewhat remarkable wall of rhyolite, rising from 1,200 to 1,800 

 feet above its base, and extending across the width of the mountains, at 

 right angles to the general trend. This wall divides the mountains into two 

 distinct groups; the portion to the north being not only much lower, but 

 presenting, for a rhyolite formation, a remarkably broad level country with 

 few prominent landmarks; the lowest portion lying directly along the base 

 of the wall, and affording an easy pass across the range. 



Although the main portion of the mountains consists of Tertiary erup- 

 tive rocks, earlier formations are exposed along the western foot-hills, which 

 derive their chief interest, and a very important one, from indicating clearly 

 the existence of an older line of upheaval, along which the later volcanic 

 rocks have found vent, reaching the surface in such vast accumulations as 

 to conceal, except over a very small area, the underlying ridges. It is evi- 

 dent, in looking at the geological map and studying these older exposures, 

 that the Tertiary outflows must have followed the lines of pre-existing 



