42 LAKES OF NOIITH AMERICA. 



somewhat extended observations made in this connection in America. 

 In fact, the blue and yellowish tints of such deposits are so general in this 

 country that the reverse of the proposition .referred to might be more 

 reasonably claimed. 



In small lakes, when sedimentation is retarded, the growth of mol- 

 lusks, diatoms, etc., may progress rapidly and their dead shells accumu- 

 late on the bottom so as to exceed the amount of mechanical sediment, 

 and shell marl and diatomaceous earth be formed. This process is 

 especially well marked in lakes that are surrounded by matted vegetation 

 through which the inflowing waters percolate and are filtered of nearly 

 all material in suspension. As the growing mosses encroach on lakes of 

 this character, a layer of peat is formed above the marl and a well-marked 

 stratification results. Llayers of peat above strata of shell marl may be 

 seen in process of accumulation in many of the small lakes of Michigan 

 and other similar regions. In lake and swamp deposits that are now 

 drained and utilized for farming purposes? a layer of white marl beneath 

 black humus, is frequently exposed. These deposits have an additional 

 interest from the fact that we find in them the bones of the mastodon, 

 mammoth, giant beaver and huge sloth-like animals that roamed over 

 North America in recent times, but are now extinct. 



