THE TERTIARY PERIOD. 251 



mammals, but none, except the marsh-loving beaver and rhinoceros 

 have yet been found. Leidy has suggested that the explanation 

 might be found in the probability that these old lakes were occa- 

 sionally flooded by the sea, producing an alternation of salt, brack- 

 sh and fresh water conditions. If that had been the case, it appears 

 to me that some indications of destroyed marine life would have 

 been detected. I suggest that the explanation may be found in the 

 probability that the waters of these lakes were too alkaline to be ^ 

 favorable to life. The deposits from these lakes indicate an exces- 

 sively alkaline condition of the water. They are rarely found free 

 from the alkaline earths, and many of the strata are almost wholly 

 composed of them. Thick beds of almost pure marl abound not 

 marl produced by beds of shells, but by precipitation from water 

 supersaturated at certain times with alkaline matters. Such waters 

 certainly were most unfavorable to animal life, and accounts for the 

 paucity of fresh water remains. 



