J. ECCLES ON THE VOLCANIC KOCKS OF MONTANA, U.S.A. 401 



' East of the Firehole river and across the divide between the 

 Madison and Yellowstone rivers the section presents the same gene- 

 ral characters as that between the Snake and the Madison. High 

 up on the divide the trachytes are light in colour, and there is some 

 pumice occurring on the surface. On the east of the divide the 

 obsidian thins away for some distance, but is found again above 

 the Great Canon of the Yellowstone, where it is capped by fully 

 300 feet of fine sandstones and shales, probably of Quaternary age, 

 which have doubtless been deposited by the Yellowstone lake in a 

 former period of extension. The rocks in this basin, and especially 

 in the section exposed in the Great Canon, have been minutely de- 

 scribed by Drs. Hayden and Peale ; and I have nothing to add to 

 their description. It is sufficient to say that the order and general 

 character of the rocks is almost precisely the same as in the upper 

 part of the Madison basin. , 



Up to this point the volcanic phenomena had been almost uni- 

 form. The trachyte -flows, although dipping slightly here and there 

 (as much as 20° in the Upper Geyser basin), were approximately 

 level ; and the country through which I had passed seemed to be 

 quite destitute of any features which I could recognize as vents from 

 which such an enormous mass could have been poured out. Seven 

 miles north of the head of the Canon a change is observable. Mount 

 Washburn here attains an elevation of from 2500 to 3000 feet above 

 the trachytes of the Canon, and is a true volcanic cone. The summit is 

 a broken-down crater; and the lava-flows, which are basaltic (No. 9), 

 dip away in all directions, and conform generally to the slopes of the 

 mountain. I found no contact between the basalts and the trachytes 

 on the southern side of the mountain ; but it is quite evident that 

 the former overlie, and are more recent than, the great mass of tra- 

 chytic rocks just described. 



Descending on the north side of the mountain, the Yellowstone 

 river was reached again near Tower Falls, a distance of ten miles 

 from the peak ; and here the trachytes are again seen in the river- 

 section, with columnar basalts resting on them. I was not sure 

 whether these basalts had flowed from Mount Washburn or from 

 some vent on the east side of the Yellowstone : but, compared with 

 the trachytes, they were comparatively insignificant both in extent 

 and, except on the mountain itself, in thickness. These basalts had 

 been poured out before the formation of the present Canon ; for the 

 river here cuts its way through basalt and trachyte nearly 400 feet 

 deep. 



The locality of Gardiner's river, whence the specimen No. 7 was 

 obtained, as well as the intervening space between this river and 

 Tower Falls, was passed over most rapidly by me, owing to bad 

 weather and Indian troubles. The trachyte in this locality resem- 

 bles strongly that of the Great Canon in mineral character. Although 

 there is a considerable distance between Tower Falls and Gardiner's 

 river where the trachyte does not exist, owing, probably, to denu- 

 dation, I am inclined to regard this Gardiner's-river trachyte as 

 belonging to the great mass of trachytes further south. 



