TERTIARY ANIMALS. 511 



in extent. Similar deposits are especially abundant in California. 

 They are found in at least a dozen localities where the Tertiary rocks 

 prevail, as, for example, at San Pablo, in Shasta County, and near 

 Monterey, the last deposit being fifty feet thick. 



Some of the more remarkable forms of Diatoms are shown in Fiof. 

 860, which is a view under the microscope of the Eichmond deposit. 



Deposits of this kind are usually called infusorial earths. They may 

 often be recognized, even without microscopic examination, by their soft, 

 chalky consistence, their insolubility in acids, and their extreme lightness. 



Origin of Infusorial Earths. — It is well known that mud composed 

 of diatom shells accumulates at the bottoms of ponds, and lakes, and 

 sluggish streams. In the deepest parts of Lake Tahoe, where sedi- 

 ments do not reach, the ooze is composed wholly of infusorial shells. 

 It has been shown, also, by Dr. Blake,* that the deposits from hot 

 springs of California and Nevada, even where the temperature is 163° 

 to 174°, abound in Diatoms of the same species as those found in Cali- 

 fornia infusorial earths. It is probable, therefore, that many of these 

 deposits were made in hot springs and hot lakes, which, judging from 

 the volcanic activity of that time, abounded in California, then even 

 far more than now. Dr. Blake thinks the infusorial earths of Cali- 

 fornia are Miocene. In the hot-springs of Yellowstone Park deposits 

 of this kind are now forming over many square miles and are five or 

 six feet thick (page 161). 



Animals. 



As already stated, among Invertebrates there was a general similarity 

 to the present fauna. Nearly all the genera and many of the species, 

 were identical with those still living. The relation between the various 

 orders which prevail 

 now, commenced 

 then. The present 

 basis of adjustment 

 was then established. 

 Then, as now, Brachi- 

 opods and Crinoids 

 were nearly all gone, Fig. sei.-xummuiina laevigata. 



Echinoderms were 



nearly all free, and Bivalves were nearly all Lamellibranchs. Then, as 

 now, naked Cephalopods and short-tailed Crustaceans greatly predomi- 

 nated. A glance at the following figures of Tertiary shells (Figs. 862- 

 876) will show the general resemblance to those of the present seas. 



In regard to the Invertebrates, there are only three or four points of 

 sufficient importance to arrest our attention in a rapid survey. 



* American Journal of Science, Part III, vol. iv, p. 148. 



