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 j 
the shape of the bottom being very irregular, the 
depth varying from 20 to 160 fathoms. The Rhine, 
w^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 ihe mouth of the Rhine, through which space 
the fluviatile mud is always found at the bottom. 
Along this gradual slope, then, of 3 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 Bedie. 
