AGE OF MAMMALS. 263 



in lake bottoms ; or estuary that is, deposited from the 

 waters of a branch of the sea, where there was a min- 

 gling of salt and fresh waters. Some deposits occurred 

 in interior (mediterranean) .seas. There was, during a 

 part of the Tertiary age, such a sea in Europe of immense 

 extent, with vast irregular islands in it. There was oft- 

 en in this age an alternation of marine and fresh-water 

 deposits, as shown by the fossils found in the strata. 

 This is the case with the basin, so called, in which the 

 city of Paris lies. First in order we have plastic clay, a 

 fresh-water deposit. Then upon this are strata in which 

 there is a vast quantity of marine shells. This is the cal- 

 caire grassier, which is, for the most part, a granular yel- 

 lowish limestone. It is regularly bedded and jointed, so 

 that it is easy to quarry it, and it is the grand building- 

 stone of Paris. Over 500 species of marine shells have 

 been discovered in these strata. By some changes of 

 the land in some way, there was at length fresh water 

 over all that region, and in the deposits made from it 

 upon the calcaire grossier there are, accordingly, found 

 remains of fresh-water animals and plants. Again the 

 sea water was let in, and we have a marine group of 

 strata, to be followed again by fresh water and its char- 

 acteristic deposits. In these last fresh-water beds the 

 most peculiar rock is the millstone, a flinty rock full of 

 cells and winding cavities, occasioned, it is supposed, by 

 the escape of gas from the bed of the lake up through 

 the deposit while it was hardening. 



370. Areas of the Deposits. The deposits of strata in 

 this age were not made over areas of almost continental 

 extent, as was the case in the previous periods. There 

 were elevations of land here and there over the spaces 

 now occupied by the continents, of such size and arrange- 

 ment that there were lakes, and estuaries, and inland seas, 

 and on the floors and shores of these the deposits were 

 made. As these elevations multiplied and increased dur- 

 ing the progress of the age, the areas of deposition were 



