172 GEOLOGICAL SURVEY OF THE TERRITOEIES. 



spread oufc in the valley, coveriuf? a space of three or four miles. It 

 forms a hluff edge 10 to 20 feet above the level of the river. A speci- 

 men of the rock that I obtained is a black vesicular basalt containing 

 free quartz and olivine. Cn pulverizing it I obtained a dark, almost 

 black, powder from which magnetite could be separated. 



This rock is of comparatively recent date, which is evident from the 

 following reasons : 



First. The flow has taken place since the carving out of the valley. 

 It occupies the bottom of the valley, which is nearly two thousand feet 

 in depth, bordering the river like the slag poured out from a furnace. 

 The river seeu)S to be the limit of the flow, none of the rock being fouud 

 on the south side. It was probably pushed by it to the lower side of 

 tlie canon-like valley. 



Second. The subsequent erosion has been very slight. The basalt is 

 exposed in a bluff-like wall which reaches to the level of the water, 

 nothing being exposed beneath it. 



Third. The basaltic rock is destitute of vegetation and comparatively 

 free from any soil. It has the appearance of having just been poured 

 out. The period during which it was poured out is probably to be meas- 

 ured by hundreds of years, and perhaps less, rather than by longer 

 periods. 



Grand River. — Below the mouth of Eagle Eiver on the south side of 

 Grand River there is an area of volcanic rock that has been subjected 

 to considerable erosion. This area was probably once continuous with 

 that west of lioaring Fork. Between a creek, Grand Eiver and Roar- 

 ing Fork it rests mainl^^ on beds of Triassic age, forming a plateau-like 

 surface. Near the mouth of Frying-Pan Creek is a mesa capped with 

 basalt which is probably a portion of the same flow. This latter, how- 

 ever, rests on beds of Cretaceous age, as does the volcanic rock west of 

 tlie Hog-backs on Roaring Fork. They are all probably remnants of the 

 same flow. The amount of erosion previous to the spreading out of this 

 material was very great. The Cretaceous rocks near the Grand, between 

 Roaring Fork and a creek, seem to have been entirely removed. The 

 subsequent erosion also has been of great extent. The present valleys 

 and caiions have probably been outlined since, and the amount of de- 

 nudation is to be measured by their depth below the level of the vol- 

 canic rock. 



The capping of the hills west of Roaring Fork is very irregular. 

 There remain only the remnants of what was once a connected mass. 

 Station 16 was located on the western edge of one of the patches of 

 basalt. 



In almost all of the valleys drained by the southern branches of the 

 Grand, between Roaring Fork and the plateau of station No. 48, there 

 are great numbers of volcanic bowlders, derived fropa the hills that are 

 capped irregularly with basalt. Tte hills themselves are so covered 

 with debris that it is difficult to define the" boundaries of the basalt. 

 The plateau on which station 48 is located is also capped irregularly 

 with basalt. Since the flow it has been subjected to a great deal of 

 erosion, and now the basalt is found only in isolated masses, like that 

 on which we made station 48. The latter is a mammallary process 

 rising 248 feet above the general level, and about 200 feet in diameter. 



The following is the section of this curious horn-like point, which can 

 be seen from a great distance in every direction : * 



1. Dark-gray basalt, containing a large quantity of olivine, also free 

 quartz sparingly. It is a very hard and compact rock. 



2. Purplish basalt. This layer is slightly vesicular. It contains the 

 same minerals that are seen in No. 1. 



