90 The Shoshone Island. 



western spur and slopes of the island, the Sho- 

 shone island of the early Tertiaries. True, it 

 to-day lacks the palm tree on the shore and 

 the rhinoceros and hippopotamus in its wa- 

 ters, but their graves are here, monuments 

 that speak eloquently of other times. 



While these sediments of mud or sand or 

 ash all come from the wear and wash of the 

 land transported into the lakes by winds or 

 streams, there is another sedimentary deposit 

 indigenous to the lake beds themselves, and 

 so different from the others as to call for a 

 word of special mention. It is known as 

 Diatomaceous earth. It is found in the form 

 of a stratified white, or yellowish white, rock, 

 often so light as to float on water and some- 

 times mistaken for chalk. It is often sold in 

 the shops under different names as a polishing 

 powder and as such is often named Tripolite. 

 On examination under a good microscope it 

 is found to be made up of the remains of one- 

 celled plants whose cell coating is silica, and 

 it is the accumulation of these shells or coat- 

 ings that makes the mass of the rock. 



