MAMMALIA ABUNDANT. 75 



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 re- 

 semblance 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 geolo- 

 gists, 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 palaontological history, 

 as something under one law, seeing that birds and mam- 

 malia, the next classes in the vertebrate scale, are then 

 added. In the words of Sir R. Murchison, who believes that 

 a true geological passage may be found between the two for- 

 mations, 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 commonly 

 called a new creation is not now maintained anywhere with 

 confidence. The more rational explanation of the appearances 

 is one suggested by actual facts observed in the strata ; that 

 the final cretaceous beds were deposited in seas more than 

 usually deep, and which were therefore no proper habitat for 

 the animals previously existing ; that an interval of time 

 afterwards took place, which is not represented by any strata 

 which have been discovered ; and that, by the time the ter- 

 tiary formation commenced, the usual modifying influences 



