158 



DELTAS IN LAKES. 



Fig. 37. 



Deltas may be divided into three kinds: 1. Those 

 which are formed in lakes ; 2. Those in inland seas ; 

 3. Those on the borders of the ocean. 



1. Deltas in Lakes. Lake of Geneva. — -This lake 

 is about 37 miles long, and from two to eight broad ; 

 the shape of the bottom being very irregular, the 

 depth varying from 20 to 160 fathoms. The Rhine, 

 v^here it enters at the upper end, is turbid and dis- 

 coloured ; but its waters, where it issues at the 

 town of Geneva, are beautifully clear and transpa- 

 rent. An ancient town, called Port Vellais, once 

 situated at the water's edge at the upper end, is 

 now more than a mile and a half inland ; this in- 

 tervening alluvial tract having been acquired in 

 about eight centuries. The remainder of the delta 

 consists of a flat alluvial plain, about five or six 

 miles in length, composed of sand and mud, a little 

 raised above the level of the river, and full of 

 marshes. In the centre of the lake, the depth of 

 the water is from 120 to 160 fathoms, and it begins 

 to grow shallower about a mile and three quarters 

 from the mouth of the Rhine, through which space 

 the fluviatile mud is always found at the bottom. 

 Along this gradual slope, then, of 2 miles in length, 

 the alluvial deposites are made ; and if we could 

 obtain a section of the accumulation formed in the 

 last 800 years,* we should see a series of strata, 



* De la Beche. 



