From Eocene to Recent Time 245 



That active land denudation and resedimentation were 

 frequent is shown by some lacustrine deposits of the period 

 that vary from 2000 to 12,000 feet In thickness. These 

 lacustrine deposits evidently originated in many cases from 

 temporary elevation of marine areas, and conversion of 

 these into lakes which received the waters of rivers, and 

 became quickly stocked with a freshwater fauna. Varied 

 glimpses are furnished from several of the continents, of 

 the biological conditions that then existed around such 

 lacustrine areas. Thus in O. Heer's striking work, as 

 translated by Heywood (/^j), the following classification 

 of the rocks that are known as "the Molasse" is given, 

 along with their leading organic contents. This applies 

 to the so-called Oeningen flora and fauna that are amongst 

 the richest known; 



Miocene, (i) Oeningian or Tortonian. Upper fresh- 

 water Molasse and brown coal. 



(2) Helvetian or St. Gallen Molasse. Marine beds 

 with animals in part of Mediterranean, in part of tropical 

 fades. 



(3) Lhangian or Mayencian. Lower freshwater or 

 grey molasse but with intercalated marine beds. 



Oligocene (i) Aquitanian. Red Molasse or lower 

 brown coal, with lignite seams and abundant terrestrial 

 vegetation. 1000 m. thick in Italy, 



(2) Stampian. In Italy 600 m. thick. 



(3) Tongrian or Lower marine Molasse. Shells, num- 

 mulites etc. 2000 m. thick. 



(4) Sestian, a marine Italian series, about 20 metres 

 thick. 



Regarding the freshwater molluscs Heer remarks: 

 "All the univalve MoUusca which Inhabited the Swiss 

 Miocene forests, and the bivalves which peopled the brooks 

 and lakes, belong to living genera. The species however 

 are almost entirely extinct; and their nearest allies are no 

 longer inhabitants of Switzerland." Snails were very 

 numerous, and include Pupa, Planorhis, Limnaea, Palti- 

 dina and Melania. As to the species Melania escheri he 

 notes that its nearest living allies are In the rivers of tropical 

 Asia. Numerous arachnids, also 844 species of insects that 



