MAMMALIA ABUNDANT. 87 



still in progress. Though comparatively a local formation, it 

 is not of the less importance as a record of the condition of 

 the earth during a certain period. 



The hollows filled by the tertiary formation must be con- 

 sidered as the beds of estuaries and gulfs, left at the conclusion 

 of the cretaceous period. We have seen that an estuary, either 

 by the drifting up of its mouth, or a change of level in that 

 quarter, may be supposed to have become an inland sheet of 

 water, and that, by another change of the reverse kind, it may 

 be supposed to have become an estuary again. Such changes 

 the Paris basin appears to have undergone oftener than once ; 

 for, first, we have there a fresh-water formation of clay and 

 limestone beds j then a marine-limestone formation ; next, a 

 second fresh-water formation, in which the material of the 

 celebrated plaster of Paris (gypsum) is included ; then a second 

 marine formation of sandy and limy beds ; and finally, a third 

 series of fresh -water strata. Such alternations occur in other 

 examples of the tertiary formation likewise. 



The end of the Secondary Formation, which we have just 

 seen take place, presents in some respects a remarkable resem- 

 blance to the close of what is called the Palaeozoic period in 

 the Permian strata. Looking broadly at the specific forms of 

 the next higher strata, they appear to have undergone a total 

 change. Again do we now witness a difference of the shelly 

 cephalopoda. There is also a gradual reduction, and finally a 

 disappearance, of the specific forms of gasteropods, formerly 

 abundant. It has heretofore been a belief of geologists, that 

 at this point, as at the former, there was an entire renewal of 

 life upon our planet ; but several considerations forbid such a 

 conclusion in the second as well as in the first instance. First, 

 the specific forms are not wholly changed, for a few do pass 

 into the next higher strata. Second, there is, in the higher 

 formation, an apparent following of an order applicable to the 

 whole palceontological history, as something under one law, seeing 

 that birds and mammalia, the next classes in the vertebrate 

 scale, are then added. In the words of Sir E. Murchison, who 

 believes that a true geological passage may be found between 

 the two formations, the upper secondary rocks judging from 

 many of their generic forms " seem to have prepared the way 

 for the sequence of the tertiary strata." For these reasons, the 

 idea of an entire renovation of life at this time what is com- 

 monly called a new creation is not now maintained anywhere 

 with confidence. The more rational explanation of the ap- 



