11(3 LAKES OF NORTH AMERICA. 



Farther back still in the records of the past are other fragments of the 

 earth's history sealed up and preserved in lake deposits. The heavy beds 

 of sandstone composing the Catskill mountains, and forming a part of the 

 Devonian system, contain shells which resemble the covering of fresh- 

 water moUusks, and may indicate that the sands in which they were 

 buried are of lacustral origin. Here the evidence of terrestrial lakes 

 seems to end. What inland water bodies existed in remote Silurian, 

 Cambrian, and Algonkian times, remains to be discovered. 



