148 LAKE DEPOSITS 



fully preserved the terraces, embankments, and deltas of the various 

 stages of water, with the gravels and sands appropriate to the shal- 

 low water. The principal part of the basin is a level plain, filled to 

 a great but unknown depth with beds of clay and marl. (Fig. 53.) 

 In still more ancient lakes the terraces, embankments, and other 

 shore features have been swept away by the processes of denuda- 

 tion, but the outline of the lake may frequently be reconstructed 

 from the character of the deposits. 



b. Chemical Deposits are not common, nor of much importance 

 in fresh lakes. In a few, chemically precipitated carbonate of 

 lime is found, and more abundant is limonite (Fe 2 3 , H 2 0). This 

 is carried into the lake by streams that contain dissolved ferrous 

 carbonate (FeC0 3 ), which, becoming oxjdized andjiydrated, is no 

 longer soluble, and accumulates on the bottom. In Sweden ores 

 of this kind are dredged out of the lakes and employed as a 

 source of iron. 



c. Organic Deposits are seldom important in large lakes, but 

 often decidedly so in small ones. As we have already seen, peat 

 often forms to such an extent as to choke up the lake and convert 

 it into a bog. Siliceous accumulations are made on an extensive 

 scale by the minute plants, diatoms, which though of microscopic 

 size, yet multiply with extraordinary rapidity ; their tests of trans- 

 parent flint gather on many lake bottoms in a fine deposit, as 

 white as flour, and variously called Tripoli, or polishing powder, or 

 infusorial earth. Calcareous accumulations are formed by the 

 shells of fresh-water molluscs, often in masses of considerable thick- 

 ness. The lower layers of this shell marl have generally been so 

 much disintegrated by the water as to be without any obvious or- 

 ganic structure. Such marls are frequently found under peat bogs 

 and indicate that the latter were originally lakes, and in the marl 

 often occur the bones of extinct animals. 



2. Salt Lakes are especially characteristic of arid climates in 

 which the rainfall is light and evaporation great. They may be 

 formed either through the separation of bodies of water from the 

 sea, or by the long-continued concentration of river water in 

 basins that have no outlet, where the influx of water is disposed 



