FISH CREEK MOUNTAINS. 665 



fine ash and earthy beds colored by oxide of iron. Skirting the western 

 base of the table are a number of low rhyolitic hills and cones, scattered 

 about in a most irregular manner, which are evidently of later origin than 

 the great body of the rhyolite table. They appear to be local centres of 

 eruption, crater-cones through which the last remnants of volcanic material 

 have reached the surface. Two such isolated cones, rising out of the 

 Quaternary plains, one of them nearly 300 feet in height, form prominent 

 landmarks, a sort of gateway to the entrance of Dacie Canon, and consist 

 of light-gray rhyolitic ash and pumice, held together in a confused manner 

 by a feldspathic binding material. Among these cones bordering the edge 

 of the valley are a number of basaltic eruptions, rarely more than 200 or 

 300 feet in height, which are evidently later than the rhyolites, breaking 

 through them and forming hard overlying beds. The relation of the 

 basaltic outbursts to the immense mass of rhyolite is very interesting, as 

 they play so small a part in the volcanic activity of the Fish Creek Mount- 

 ains, only reaching the surface at intervals along a low narrow belt about 7 

 or 8 miles in length, directly superimposed upon the acidic rocks, and 

 inclined gently toward the valley. 



Nowhere is the later age of the basalt more strikingly shown than in two 

 prominent crater-cones rising out of the volcanic debris slope about 5 or G 

 miles to the southward of the extreme northern end of the mountains. These 

 cones present a very symmetrical outline, with a nearly circular base, and 

 steep, but regular, slopes. The lower two-thirds of the cones consist of 

 light-gray rhyolite, while the summit is composed of dark basaltic rock, 

 which, after the rhyolite had ceased to pour out, reached the surface through 

 the same vents, building up the cone originally formed by the earlier flows. 

 The contrast between the two varieties of rock is very distinctly marked, 

 the basalt extending down the slope over the rhyolite in well-defined beds. 

 From the mouth of one of these craters, the last outburst has been a porous 

 basaltic lava, which has not only poured down the sides of the cone, but 

 has run out for a quarter of a mile on the plain. As the surface of the lava 

 is free from all soil and vegetation, while the surrounding rhyolitic debris 

 is partially covered with a growth of bnuch-grass and scrubby sage-brush, 

 the impression received is one of very recent volcanic activity. 



