OCEANS AND LAKES 269 



or arms of the sea are found interbedded with other rocks 

 in various regions. Those in central New York may have 

 an area underground of some 10,000 square miles (larger 

 than Vermont), and individual beds are in places 80 feet 

 thick. To form a layer of salt 80 feet in thickness would 

 require the evaporation of some 6000 feet of sea water of 

 average salinity. Clearly, the climate of New York must 

 have been much less humid than now at the time the salt 

 was deposited. Salt beds, like other deposits, therefore aid 

 in determining the geography of the past. Their presence 

 points to a period of aridity ; their thickness suggests some- 

 thing of its duration. 



Other chemical deposits. While the deposits mentioned 

 in the preceding paragraphs are perhaps the most common 

 ones, many other substances are precipitated from the 

 waters of certain lakes. For example, iron is precipitated 

 so abundantly in some of the lakes of Sweden that it is of 

 commercial value, and in the colonial period it was dredged 

 from certain of the morainic lakes in eastern Massa- 

 chusetts. 



Marl. Marl is a soft, limy clay, formed principally on 

 lake bottoms. The calcium carbonate is contributed by 

 the shells of fresh-water mollusks, by the decay of lime- 

 secreting lake plants, and possibly in some instances by 

 chemical precipitation. Only where little clay is washed 

 from the slopes tributary to the lake is marl formed. 

 Marl is used extensively for the manufacture of Portland 

 cement. 



Peat. The formation of peat in flood-plain marshes has 

 been noted (p. 178). Extensive peat deposits have been 

 made also in shallow lakes and in the marshes which in 

 many cases replace them. Peat deposits are most ex- 

 tensive in regions having moist and relatively cool climates, 

 though by no means confined to them. A moist climate 

 favors a heavy vegetation, and a cool climate retards its 

 decay. 



