34 GEOLOGICAL SURVEY OF THE TERRITORIES. 



The process of wearing out this channel for three miles in length 

 through such a thickness of hard limestone must have required ages, 

 sufficient for the waters of the lake to have deposited 1,000 to 1,500 feet 

 of sediment, and, as the channel was cut down and the basin drained, 

 a portion of the sediment would be swept down tlie river. We have 

 now the evidence that the waters of the lake must have reached high 

 up on the sides of the mountains, entering far up the open side-valleys, 

 in some cases nearly up to the divide or water-shed. The line of demar- 

 cation between the modern deposits as they jut up against the mount- 

 ain-sides, and the natural debris of the mountains themselves, is quite 

 distinct, and is even shown by the vegetation. When we reflect that 

 the productiveness, as well as the possible settlement of these mountain- 

 regions, is due to the former existence of these lakes, we shall at once 

 understand their importance, and their history will become invested 

 with a greater interest. It is only in these valleys that farming-lands 

 can be found. The sediments which were accumulated in the bottoms 

 of the lakes were derived from the destruction of a great variety of 

 rocks, so that the mixture is m'ost remarkable for its fertility. The 

 metamorphic and igneous rocks, and the limestones of the Silurian aad 

 Carboniferous epochs have all contributed to them. As the lakes were 

 drained slowly away, the bottoms were worn out and smoothed as we 

 see them now. Here and there we find that these superficial deposits 

 have been stripped off, so as to expose remnants of the old formations 

 which constituted the original skeleton. Patches of limestone are 

 observed here and there, enough to indicate something in regard to the 

 former history of this surface, or skeleton, as it might be called. 



Although this valley was originally largely due to erosion no doubt, 

 yet it was not altogether so. It was not a chasm or a fissure in which 

 the waters gained a foothold for their operations, as was the case with 

 many of the valleys. The main feature of the mountain-range, as we 

 see it now on the east side of the Yellowstone Eiver, never crossed to 

 the west, side, but formed the east shore of the lake. That the valley 

 was greatly enlarged by the wearing away of the sides of the mount- 

 ains by the waters of the lake, there is little room to doubt. On the 

 west side of the valley the mountains rise up 9,000 and 10,000 feet above 

 the sea-level, but are mostly volcanic and most probably conceal a vast 

 thickness of sedimentary beds. The igneous rocks seem to have issued 

 from numerous fissures, and to have spread over the surface to an enor- 

 mous thickness. But the rocks which prevail over all the rest are those 

 which have been formed out of fragments, dust, ashes, &c., which must 

 have been thrown out of the numberless volcanic craters into the sur- 

 rounding waters, and been afterward deposited as sedimentary beds. 

 The massive basalts or trachytes may be considered the exception, 

 while we find 2,000 to 4,000 feet in thickness, of volcanic breccia or 

 conglomerate, reaching to the very summits of the highest mountains, 

 and presenting a well-marked horizontal stratification. Materials of 

 almost ev^ery variety of color, with a peculiar somber hue, are found. 

 Immense masses have fallen down into the valley from the mountain- 

 sides, composed of fragments of trachj^te of every possible texture and 

 color. Sometimes these fragments are very coarse, several feet in diam- 

 eter, and again they are small, like pudding-stones. Sometimes they 

 are angular as if they had not been .subject to erosion in water, and 

 again they are much rounded. The cement is also more or less fine 

 volcanic material, partly, perhaps, the dust and ashes thrown out of the 

 volcanic fissures, and partly the eroded materials from the rounded 

 fragments. In the stratified beds there is also a great variety of mate- 



