The Shoshone Island. 87 



The black stubs of inky shade are so many 

 dykes of eruptive rock burst through the 

 mass. One of these, a dyke of columnar 

 basalt, is exceptionally instructive. Seen at 

 the distance of two or three miles it seems a 

 vast wood pile a giant's wood pile. Seen at 

 closer range it is found to be a stack of rock 

 prisms of great regularity, evidently once 

 forced up through this old lake sediment, from 

 which atmospheric wearing has long since 

 cleared away the softer covering, leaving our 

 wood pile a picture itself. 



These eruptive rocks in their relation to 

 the general landscape seem like ink blotches 

 on the face of a painting. If we look care- 

 fully at the other rocks we shall find them all 

 hardened sediments, fresh water sediments, 

 for whenever they contain shells these are 

 fresh water, or land shells, but never marine. 



The aggregate thickness of these sedi- 

 mentary rocks of Tertiary times, between the 

 Shoshone island and the Cascade mountains, 

 is very great not less than three thousand 

 feet. 



