TERTIARY STRATA OF OTHER REGIONS. 



377 



skeletons or cases of Infusoria. A stratum twenty-eight 

 feet in thickness, interposed between the eocene and miocene 

 stages in North America, and extending over a considerable 



FIG. 266. 



area, the city of Eichmond, in Virginia, being built on it, is 

 composed of fossil infusoria, chiefly of the genera Navicula 

 and Gaillonella. The remarkable composition of these rocks 

 tends strongly to confirm the supposition of the organic 

 origin of many deposits. 



We have thus brought to a close our remarks on this 

 instructive period of the earth's history. Our observations, 

 however incomplete, when compared with the magnitude 

 of the subject, are more in detail than our limits will 

 permit us to devote to the other formations, as many of the 

 phenomena observable in the tertiary are repeated in the 

 older rocks ; the explanation of these, in the first instance, 

 will obviate the necessity of their repetition in succeeding 

 chapters. 



The deposition of the tertiary strata in depressions of the 

 chalk is demonstrated by the basins of London, Hampshire, 

 and Paris; the changes in the level of land and sea are 

 proved by the alternations of fresh- water and marine strata': 



