NAMES OF THE GEOUPS. 39 



Colorado sea similar to those in the New York sea, is not warranted by a 

 study of the deposits now forming in existing seas. The Hudson River 

 carries a very different deposit into the Atlantic Ocean from that carried by 

 the Colorado River into the Gulf of California. Nor should we expect that 

 the faunas or floras of regions so widely separated should be the same or 

 closely similar. In the earlier times, which we study as geologists, there 

 seem to have been physical conditions in the two regions as widely differing 

 as those of the present. The Cenozoic formations of the plateaus are lacus- 

 trine ; the Cenozoic formations of the Atlantic slope are marine. The Meso- 

 zoic of the plateaus is of great extent and thickness, while that age is but 

 scantily represented on the Atlantic slope. Nor do the Paleozoic forma- 

 tions exhibit a close similarity. 



The names which I have selected for the groups are geographic, as 

 such a system admits of easy interpolation, and the localities serve well in 

 fitting the name to the group and refer at once to the typical strata. For 

 obvious reasons I should have been pleased to have commenced with a 

 clean slate, selecting such localities as would serve for the best types ; but 

 I did not feel at liberty to ignore the labors of geologists who had pre- 

 ceded me. 



In the Cenozoic groups and the first Mesozoic I found great confusion, 

 as these groups had been seen at many places, and some of them received 

 several names each ; and often different groups were confounded by being 

 included under one geographic name. With these groups I have tried to 

 select such localities as would serve to fairly represent the groups and at 

 the same time do no injustice to other laborers. 



We now append the general section. 



