64 GEOLOGICAL SURVEY OF THE TERRITORIES. 



and the most beautiful spring iu the Upper Geyser Basin of the Madi- 

 son. The Grotto and Giant Geysers are first-class spouters, throw- 

 ing a column of water up from 100 to 200 feet and continuing the oper- 

 ation from one to tvTo hours. The "Boiling Spring," at Sulphur Moun- 

 tain, is located in the valley of the Yellowstone, about tea miles below 

 the lake, at a locality known as the " Seven Hills." It is indicated on 

 the chart in the report of 1871, i)age 88, and described on page 89. This 

 spring is known as the Sulphur Spring, and the open portion is about 

 15 feet in diameter. The ornamentation about this spring is beautiful 

 in the extreme. At the lower right-hand corner is a small sketch of the 

 Calcareous Springs on Gardner's Eiver, with the curious terraces and 

 bathing-pools which were described in detail last season. 



So far as we could ascertain in our explorations, all the rocks about 

 the sources of the Yellowstone and the Madison are of volcanic origin. 

 It is true that in the valleys, the very modern lake-deposits reach a 

 thickness of several hundred feet, but they are seldom taken into 

 account as distinct geological formations. Still, they are found every- 

 where in the valleys. The foot-hills that border the Yellowstone Valley 

 between the falls and the lake are entirely made up of them, and must 

 have an aggregate thickness of 600 to 800 leet at least. Between the 

 Yellowstone Valley and the head-waters of Madison and Snake Elvers, 

 these modern deposits are seen everywhere, filling up the inequalities 

 of the surface of the underlying volcanic rocks, and giving a rounded 

 smoothness to the present surface. They form the soil upon which the 

 existing vegetation grows. Tliese deposits are all plainly local in their 

 origin ; that is, they are derived from the rocks in the immediate vicin- 

 ity, or within the limits of the drainage in W'hich the specific beds are 

 found. For example, the modern lake- deposits of the Yellowstone 

 Basin were all derived, so far as I could observe, from the rocks within 

 the limits of the drainage of the Yellowstone Eiver above the falls. 

 The local origin of the modern drift-deposits seems everywhere appar- 

 ent. The same deposits are found to a less extent in the Geyser Basins 

 of the Madison. On the west side of the Madison, above and below the 

 junction of East Fork, the bluff bank is 400 to 500 feet high, composed 

 of trachyte. In the basin are a number of long, low ridges or hills, 

 composed of porphyritic obsidian and trachyte, which, I think, are only 

 the remnants of the great continuous mass out of which the entire valley 

 has been carved. The Twin Buttes are also partially-disconnected por- 

 tions nearer the high trachyte ranges that border the basin. Much of 

 the rock is made up of rounded masses with a radiate structure, as if 

 the igneous rocks were partially crystalline ; other masses are com- 

 posed of concentric coats, with cavities filled with feldspar, which, 

 decomposing readily, give to the extensive surface a rough appearance. 

 From the junction of East Fork down six miles, the Madison flows very 

 majestically and most beautifully. The banks are low, and fringed with 

 vegetation of the most vivid green, while visible through the water, the 

 patches of vegetation give a most pleasing variety to the current. The 

 warmth of the water seems to have given a kind of tropical luxuriance to 

 the vegetation, stimulating to an unnatural growth. There comes a series 

 of rapids of great beauty, with walls on the west side 600 to 800 feet, 

 rising nearly vertically, and composed of layers of trachyte. Much of 

 it is made up of rounded masses from the size of a pea to several inches 

 in diameter, either with a semi-crystalline radiate structure, or filled 

 inside with feldspar. The east side of the Madison recedes in step-like 

 hills or ridges, until a certain elevation is rea(ihed, when, as far as the 

 eye can reach, nothing caa be seen but a dense forest of pines. We 



