*34 



PHYSICAL GEOLOGY 



Lakes equalize the flow of streams, preventing floods, and also act 

 as filters. 



Chemical Deposits. — In addition to such mechanical deposits as 

 those described, chemical deposits are also found in lakes. Lime is 

 sometimes deposited, and iron in the form of limonite (p. 686) is 

 precipitated. In some of the lakes of Sweden and Canada iron of 

 this origin is so abundant as to be of economic importance. 



Organic Deposits, (a) Diatoms. — Dredgings in lakes show that 

 the bottoms are sometimes covered with thick deposits of diatoms 

 (microscopic plants which secrete siliceous tests, p. 581). Since these 

 organisms multiply with great rapidity, they may form extensive 

 deposits, called diatomaceous earth. 



(b) Marl. — Calcareous deposits in the form of marl may accu- 

 mulate to great depths in lakes. This is a white, or gray, clay-like 

 deposit which is composed largely of calcium carbonate. It is formed 

 either by the accumulation of shells, or through the agency of certain 

 plants (algae) which extract carbon dioxide from the water and thus 

 cause the deposition of the lime dissolved in the water. Marl is 

 formed only where small quantities of clay are washed into the lake, 

 since, if large quantities are carried in, the deposit would be termed 

 mud. Deposits of marl may be a score or more feet in depth and are 

 often overlain by peat. In regions where limestone is not accessible, 

 marl is sometimes used in the manufacture of Portland cement. 



(c) Peat. — A brown deposit, called peat, composed of the partially 

 decayed remains of plants, sometimes accumulates in swamps, 

 marshes, and shallow lakes. Peat forms most rapidly in cool, moist 

 climates where, although the vegetation may not grow rapidly, the 

 low temperature retards decay. Under favorable conditions it also 

 accumulates in warm countries. In Florida, for example, there are 

 considerable areas of peat. Extensive areas of peat occur in the 

 United States, such as that of the Dismal Swamp of Virginia and 

 North Carolina. In Massachusetts, it is estimated that there are 

 15,000,000 cubic feet of peat. One tenth of the surface of Ireland is 

 underlain by peat, and large areas in Europe and elsewhere are pro- 

 vided with it. Peat is dried and used for fuel in some regions where 

 it occurs in great abundance, and where its extraction is easy. 



Playas. — In desert regions, where no permanent lakes occur, 

 streams sometimes reach depressions when their volumes are increased 

 during the wet season or by cloud-bursts, and form temporary, 

 shallow lakes which may cover large areas. The largest in Nevada 



