THE VOLCANIC PEAKS. 189 



panion of the granite of the Harney Peak region, but also a few fragments 

 of the granite itself. It is thus known not only that the uplift was in 

 existence at a time near the beginning of the White River lake period, but 

 also that it had been so long in existence as to permit the wearing away of 

 its upper portions so as to lay bare the granitic nucleus. 



It is worthy of remark that while the deposits of the Cretaceous, from 

 the demolition of which the White River Tertiaries were evidently largely 

 formed, are dark, carbonaceous clays or shales, the latter are notable for 

 their light color. This is probably due to the making over of the sediment 

 in shallow water, whereby the carbonaceous matter was oxidized, leaving 

 the accumulation nearly white in color. A similar phenomenon is to be 

 seen in the work of the modern streams. The two branches of the Chey- 

 enne, which flow such long distances through the Cretaceous and derive 

 their drainage largely from its clays, always transport sediment of a light 

 color. Held in suspense it gives the water a milky color, and precipitated 

 on the bottom it appears as a white scum. Left on the banks by higher 

 stages of the water, it takes the form of a light colored clayey material which 

 dries in the sun to a hard white crust. So fine is it that on a windy day 

 large white clouds of the material can be seen driving along the valley like 

 clouds of smokes. The rationale appears to be that the finely divided silt 

 derived from the Cretaceous clays has by constant exposure to the air had 

 its carbonaceous coloring matter oxidized and burnt out. 



SECTION X. 

 THE VOLCANIC ROCKS AND VOLCANIC PEAKS. 



Frequent allusion has already been made to the volcanic peaks. Indi- 

 vidually they are small in extent, but they derive importance from their 

 number and from the fact that their uniformity of character over a large 

 area shows the wide distribution of the conditions necessary to their pro- 

 duction. 



