44 GEOLOGICAL SURVEY OF THE TEEEITOEIES. 



and had gradually elevated the western portion of the continent to its 

 present position, obtained vent at this time, and exhausted themselves, 

 leaving behind them stupendous monuments of their power. I am now 

 inclined to believe that when our western country is more thoroughly 

 explored by competent geologists, it will be found that the area covered 

 with volcanic rocks is far greater than we have hitherto suspected. 

 Like the more modern Tertiary beds, tlie basalts and conglomerates of 

 volcanic origin have been subjected to terrific erosion, and only a por- 

 tion of their wonderful magnitude is left behind. 



So far as my own explorations have extended, the main portion of the 

 volcanic material of tbe West has been thrown out at a comparatively 

 modern date. Among the Cretaceous and coal-bearing groups are irreg- 

 ular interstratified beds of basalt, but the great mass of trachyte, basalt, 

 and volcanic conglomerates seems to have been erupted since the surface 

 attained pretty nearly its present configuration. The conglomerates, 

 although in some instances reaching a thickness of 3,000 feet at least, 

 do not seem t3 have been disturbed to any great extent since their de- 

 position. The position of the trachytes, which have overflowed the 

 mountain ranges, indicates that they could not have been very ancient, 

 perhaps not older than later Miocene or early Pliocene, while the true 

 basalts are extremely modern, approaching closely to our present era. 

 In the report of last year I described the modern basaltic outflows on 

 both sides of the Yellowstone, above and below the junction of the 

 East Fork. On the sides of the granitic mountains, at different eleva 

 tions, the black igneous outflows can be seen, looking like hot spring-de- 

 posits, were it not for their dark color. The liquid material seems to have 

 oozed out from fissures in the metamorphic rocks in numerous places. 

 These basalts fracture readily into small fragments, and the debris re- 

 semble a pile of dull anthracite coal. These very modern basalts seem 

 also to have oozed up through the fissures in the Cretaceous and Ter- 

 tiary beds, as shown on the east side of the Yellowstone opposite the 

 mouth of Gardiner's River. Here the outcropping edges of several hun- 

 dred feet of strata are shown for some miles, spotted with the patches of 

 the black dehris^ from the breaking in pieces of the basalt that had 

 flowed out at different points. We may, therefore, conclude that this 

 period of intense volcanic activity probably commenced somewhere dur- 

 ing the later Miocene or early Pliocene epoch, reached its greatest 

 power, and then slowly declined, the hot springs and geysers of the 

 present time being the faint departing remnants of these once terrific 

 forces. 



CHAPTER III. 



FROM EAST FORK TO THE MINING DISTRICT ON CLARK'S 

 FORK AND RETURN— YELLOWSTONE VALLEY AND HOT 

 SPRINGS— GEYSER BASINS AND MADISON RIVER. 



On the 1st day of August I started from the forks of the East Fork 

 for the Clark's Fork mines, in company with Mr. William Blackmore, 

 and Mr. Holmes, artist of the survey. The camp was stationed down 

 below the bridge in a sheltered valley, where the animals would be 

 secure from danger and get a good supply of food. This trip was to 

 take us over new ground that had been omitted in our explorations of 

 last year. Our course was up the valley of the middle branch, past Soda 



