270 GEOLOGY. 



out in search of its food.* " It is evident," says Mr. 

 Eley, an English geologist, " that the larger of the an- 

 cient lakes of Auvergne was inhabited by a species of 

 this family, and that they swarmed in it in a remarkable 

 manner ; for their cases, incrusted with a calcareous mat- 

 ter, are seen to form there thick layers of limestone 

 called indusial, from this strange origin which altern- 

 ate with the more usual kinds of marl through a thick- 

 ness of several hundred feet." These indusial strata are 

 eight or ten feet thick, and extend over an area of many 

 square miles. When you are told that one of the cases 

 or tubes contains over a hundred shells, and ten or twelve 

 tubes may be counted in a single cubic inch of rock, you 

 may have some idea of the countless myriads of minute 

 mollusks which must have formerly lived and died in ev- 

 ery part of this region, and of the length of time required 

 for the formation of the strata constructed chiefly from 

 their shells. 



380. Mammals. Animals of this class were specially 

 prominent in the scenes of this age. Tapir-like pachy- 

 derms flourished largely in the early part of the age, but 

 as we come to its latter portions in the examination of 

 the strata, we find the mammals more like those of the 

 present age. Still, there is not a single species of all 

 those that have this resemblance which is not now ex- 

 tinct. There are some localities that are peculiarly rich 

 in remains of the mammals of the Tertiary. In the Up- 

 per Missouri region, in this country, among other re- 

 mains, there have been found those of three species of 

 camel, a rhinoceros as large as the Indian species, a mas- 

 todon, an elephant, a wolf larger than any species of the 

 present day, four or five species of the Horse family, etc. 

 It seems strange to us that some of these animals should 

 exist in such a locality, so far from the haunts of similar 

 animals in the present period ; but more strange still is 



* For a full description of the habits of these animals, see my Nat- 

 ural History, 459. 



