
- 
Parr II. Seor. ii. § 4.] FRESH-WATER LAKES. 391 . 
(g) Sea-borne Sediment.—Although more properly to be noticed 
- under. the section on the Sea, the final course of the materials worn 
by rains and rivers from the surface of the land may be referred to 
here. By far the larger part of these materials sinks to the bottom 
Fig. 130.—DELTA oF THE GANGES AND BRAHMAPUTRA (WITH SCALE OF MILEs). 
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at 
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close to the land. It is only the fine mud carried in suspension in 
the water which is carried out tosea. The sea fronting the Amazon is 
discoloured for 300 miles by the mud of that river. The soundings ~ 
taken by the “Challenger” brought up land-derived detritus from 
(e438 4 1500 fathoms,—200 miles or more from the nearest shores 
p. 438). 
§ 4. Lakes. 
Depressions filled with water on the surface of the land, and 
known as lakes, occur abundantly in the northern parts of both 
hemispheres, and more sparingly, but often of large size, in warmer 
latitudes. They do not belong to the normal system of erosion in 
which running water is the prime agent, and to which the excavation 
of valleys and ravines must be attributed. On the contrary, they are 
exceptional to that system, for the constant tendency of running 
water is to fillthem up. Their origin, therefore, must be sought among 
some of the other geological processes. (See Book VIL) 
Lakes are conveniently classed as fresh or salt. ‘hose which 
possess an outlet contain in almost all cases fresh water; those which 
have none are usually salt. 
I. Fresh-water Lakes.—In the northern parts of Hurope and 
America, lakes are prodigiously abundant on ice-worn rocky surfaces 
irrespective of dominant lines of drainage. ‘They seem to be distri- 
buted as it were at random, being found now on the summits of ridges, 
