GEOLOGICAL SURVEY OF THE TERRITORIES. 



131 



this point. The carving out of the Grand Caiion (Fig. 33) gives ns an 

 excellent opportunity to study the various roclvs that underlie the valley. 



At the head of the Grand Caiion is the lower fall of the Yellowstone, 

 ■which was measured this year by triangulation and found to be 397 feet 

 in height. At tlie ' p. 



foot of the fall the ^' '^^' 



depth of the canon 

 is 675 feet. This 

 depth increases as 

 ■we go down the 

 river, and about 

 half a mile belo^w 

 it is 1,000 feet, 

 measured ■with 

 an aneroid barora 

 eter. The fall of 

 the river from the 

 foot of the fall to 

 the mouth ot 

 Tower Creek, a 

 distance of twenty- 

 two miles, aver- 

 ages 40 feet to tlu 

 mile. On the west 

 ern side of th< 

 caiion, some dis 

 tance above th( 

 top of the lower 

 fall, there is an ex- 

 posure of a very 

 fine soft sandstone 

 having a light-yel- 

 yellow color. This must have been deposited in very quiet waters and 

 is jjrobably of Post-pliocene age. On the eastern side we find the rocks 

 to be as follows: The top of the caiion above the falls is made up of 

 obsidian, which is porphyritic, containing crystals of sanidine. The 

 rock is very irregular in composition and color, the latter varying from 

 black to brown. It passes into a i)eriite-like rock of a light-bluish color, 

 in some places is white. This has in places perlite-like trachyte- 

 porphyry, containing small feldspathic balls ■s\nth a radiated fibrous 

 structure, (spherulites,) mixed with small bits of obsidian, the whole 

 mass having a general color that resembles blue lead. A little farther 

 down we find in this same rock jasper-geodes containing varieties of 

 opal in the interior. They vary in size, some being only an inch in 

 diameter, while others are half a foot. They are of a brown color on the 

 exterior and botryoidal in shape, and are porphyritic, containing crystals 

 of sanidine. Breaking them open ■we find that the mass often presents 

 a beautiful appearance, some of the specimens having a rich-brown 

 color, mingled with bright red and green, ■while the cavities are lined 

 ■with pink, white, or blue semi-opal in some cases and in others by 

 hyalite. The variety in the shades of color is almost infinite. Just 

 above the fall there is a slide of rock reaching from the top of the canon 

 to the river's edge, and here we found many good specimens that had 

 broken out of the more massive rock and fallen down. 



At the brink of the fall the rock over which the water pours, and 

 which extends some 50 or 75 feet above it, is a compact trachyte por- 



GEAND CANON OF THE YELLOWSTONE. 



