580 HISTORICAL GEOLOGY 



Diatomaceous earth resembles chalk in color, but is lighter in weight, 

 and, since it is composed of silica, does not effervesce with acids. On 

 account of the hardness of its constituent parts and its extreme fine- 

 ness, it is used as a base in the manufacture of preparations for clean- 

 ing and polishing silver, nickel, etc. 1 Since it is porous, it has been 

 used as an absorbent for nitroglycerin in the manufacture of dyna- 

 mite. It is also used as a non-conductor of heat. 



The valuable phosphate deposits of Florida are believed by some 

 investigators to have originated by the leaching of guano, or bone 

 beds, and the deposition of the phosphate in the underlying limestone, 

 either by precipitation in the pores of the rock or by replacing the 

 limestone molecule by molecule. The phosphate may, however, have 

 been disseminated through the beds in small quantities and later 

 concentrated as the more soluble limestone was dissolved and carried 

 away. 



Western Interior. — In the Great Plains region east of the Rocky 

 Mountains, the conditions traced in the Oligocene continued, and were 

 probably not unlike those now prevalent where the flood plains of 

 the upper Paraguay, Amazon, and Orinoco rivers of South America 

 are confluent. In this portion of South America is a region larger than 

 that occupied by the Miocene deposits of North America, with all the 

 conditions necessary for the deposition and present distribution of 

 sandstones, clay, and conglomerates, together with the preservation 

 of animal remains. North American Miocene formations are found 

 from Montana into Texas, although largely covered to the south and 

 east by later deposits. Sediments of this age occur also in Montana, 

 Nevada, Colorado, Oregon, British Columbia, and Alaska. 



A lake existed in Colorado at this time (Florissant) which is inter- 

 esting because of the excellent preservation of many insects and 

 plants in its deposits. It lay in a narrow valley in the vicinity of 

 active volcanoes, whose numerous eruptions spread ashes over its 

 surface, burying the insects and plants which had been carried into it. 



Pacific Coast. — The restricted seas of the Oligocene on the Pacific 

 coast were much expanded during the Miocene, although at no time, 

 as will be seen by consulting the map (Fig. 530), was a large portion 

 of what is now land in that region submerged. The southern portion 

 of the Great Valley of California (San Joaquin) was beneath the sea 

 early in the epoch (Vaqueros), and in this bay a great thickness of 

 marine sediments, consisting of sands and clays with some conglom- 



1 Volcanic ash is also used for this purpose. 



