GEOLOGICAL SUKVEY OF THE TEEEITOEIES. 179 



Its temperature was IST*^' F. The water was acid in reaction^ and con- 

 tains — 



SulpTijate of iron and alumina, (abundant,) 

 Sulphate of magnesia, 

 Sulphate of lime, 

 Chloride of calcium, 

 Oxide of iron, 

 Free sulphtir, 

 ■ Soda gjid potassa, (trace.) 

 There is in the ravine in which the creek is situated a deposit of sul- 

 phur, and also near the spring a deposit, white in color, containing — 



Sulphur, 

 Iron, 

 Alumina, 

 Silica. 

 In the bed of the stream there is an abundant deposit of sulphur 

 and also a black carbonaceous material. The sulphurous odors ema- 

 nating from the ravine are so strong as to be recognized at a consid- 

 erable distance from it. A short distance above Tower Creek we 

 ascended a peak called Mount Washburne, whose summit is composed 

 of a light-gray trachytic rock containing acicular crystals of hornblende. 

 On the sides of the mountain we found large pieces of chalcedony and 

 agate. Near the base of the mountain there are situated quite a num- 

 ber of sulphur and mud springs. A specimen from one of the latter 

 was of an almost black color, and when dry was covered with a white 

 efflorescence. It contained — 



Sulphate of alumina and iron, 

 Sulphate of magnesia, 

 Sulphide of calcium. 

 Sulphur, 

 Silica. 



Our next camp was near the Great Fall of the Yellowstone. It is at 

 the head of the Grand Canon, a gorge averaging about a thousand feet 

 in depth, which the water has cut through the volcanic rocks. These 

 rocks are mostly trachytes of a white or gray color, on top of which 

 there is a layer that is basaltic in its character. In many i3laces they 

 become rhyolitic, and contain crystals of sanidine, very abundantly dis- 

 tributed through them. In one place I found a perlite-like trachyte 

 porphyry, containing small feldspathic balls (spherulites) with a radi- 

 ated fibrous structure, mixed with small pieces of obsidian. Some of 

 the rocks are colored by iron, which has been deposited from hot springs. 

 In other places there is an infiltration of sulphur, which gives them a 

 bright-yellow color. There are still some warm springs on the edge of 

 the river, and, at the only place we were able, to get to it, there were 

 three or four small si)rings giving off carbonic acid gas, which has 

 caused an abundant deposit of sesquioxide of iron about them. Having 

 no thermometer with us, we were unable to determine the temperature 

 of the water; but it could not have been much over 90°. It contained 

 a white organic material. Passing the upper fall, after a ride of about 

 eighteen miles, we reached Crater Hills. These consist principally of 

 two conical hills about 150 feet in height. There are several other hills 

 which are smaller. They are all made up in part of hot-spring deposit 

 and a white trachytic tufa. All about the hills there is an extensive 

 deposit, mostly siliceous, forming a crust which often breaks through 

 while walking over it. It is lined with beautiful crystals of sulphur. 



