GEOLOGICAL SURVEY OF THE TERRITORIES. 



37 



Upon this rests a huge bed of drift, which was deposited still later, and 

 then comes the outflow of basalt before the waters subsided, as is shown 

 by the texture of the rocks, as well as the superficial deposits over it ; 

 then the waters were drained slowly away, sweeping with them most of 

 the basalt, with the exception of here and there a remnant, and also most 

 of the deposits beneath, thus giving shape to the valley. All these events 

 must have taken place subsequent to the completion of the general out- 

 line of the country by the upheaval of the mountains. It is probable, 

 also, that during this period, and probably throughout the greater por- 

 tion of the Tertiary period, hot springs were very abundant everywhere, 

 and it is doubtless due to these that the organic remains found in these 

 deposits have been preserved in such a high state of perfection and 

 beauty. The silicified or opalized wood, we believe, indicates the 

 presence of these springs. There are a few warm or moderately hot 

 springs in this valley at the present time. 



As an illustration of the volcanic action in this valley, the reader is 

 referred to Fig. 2, which was sketched by Mr. Holmes, about two miles 



Fig. 2, 



TKACHYTE DIKE AND COLUMNS OF BRECCIA, YELLOWSTONE VALLEY. 



above Boteler's ranch on the west side of the Yellowstone Valley. The 

 long, wall-like mass that extends down from the side of the hill is a 

 dike of trachyte, while on cither side are the tufas and breccias, which 

 have been weathered into columns of varied fornis. This may also repre- 

 sent one of the numerous fissures or oblong craters from which much of 

 the volcanic material was ejected. Fig. 3 shows quite clearly the 

 stratified character of the tufas and breccias. This illustration was 

 taken about a mile above Fig. 2, on the same side of the Yellowstone. 

 The entire mass is most curiously variegated in color, from an ashen- 

 cream color to deep purple. This series of gray volcanic ashes alter- 

 nates with more or less coarse strata of breccias, the fragments cemented 



