THE MIGRATIONS OF MARINE ANIMALS 289 



wards its proper limits : on the blue slime are 

 superposed fine sandy deposits with a much more 

 littoral fauna, characterized by an abundance of 

 the great bivalves, Naticas, Conoids, Balanas, etc. ; 

 these are the deposits of the Astian stage. To- 

 wards the end of this stage, the retreat of the sea 

 becomes evident by an abundance of oyster beds 

 with which estuary shells, such as the Potamidae, 

 begin to be associated ; then the freshening of the 

 waters becomes more marked, and we find a second 

 lagoon phase with a fauna of Congeries and Melan- 

 opses greatly similar to that of the phase at the 

 beginning of the Pliocene. Finally, all traces of 

 brine disappear, and we see superposed on the 

 Astian layers a marl containing fresh water Mol- 

 luscs, and at length river sand and pebbles contain- 

 ing no other fossils than the bones and teeth of 

 Mastodons and other terrestrial animals. 



These phenomena taken as a whole, from the 

 irruption of the sea into the pre-Pliocene valleys 

 to the filling up of these valleys with the river 

 pebbles, constitute what may be. termed a cycle of 

 sedimentation. We should meet with a very analo- 

 gous cycle if we wrote the history of the Miocene 

 period in the same Mediterranean basin. The 

 Eocene history of the Paris basin is likewise com- 

 posed of a repeated series of similar, but less com- 

 plete, cycles, with alternations of incursion periods 

 with frankly marine faunas and of retreats of the 

 sea with brackish faunas, and with the formation 

 of lagoons in which was accumulated gypsum, a 

 product of the evaporation of sea water. 



It now becomes easy to understand why, given 

 u 



