160 
DELTAS OF INLAND SEAS. 
bottom of Lake Superior* consists of a very adhe- 
sive clay, containing shells of the species at pres- 
ent existing in the lake. When exposed to the air, 
this clay immediately becomes indurated in so great 
a degree as to require a smart blow to break it. It 
effervesces slightly with diluted nitric acid, and is 
of different colours in different pans of the lake ; in 
one district blue, in another red, and in a third 
white, hardening into a substance resembling pipe- 
clay. "These lacustrine formations," Mr. Lyell 
remarks, " resemble the tertiary argillaceous and 
calcareous marls of lacustrine origin in Central 
France, as many of the genera of shells are the 
same." 
Deltas of Inland Seas. — It is maintained by Mr. 
Lyell, that the rapid gain of land in the Baltic 
Ocean, especially the gulfs of Bothnia and Finland, 
is not only owing to the influx of sediment from 
numerous rivers, but also to a slow and general up- 
ward movement of the land itself, at the rate of 
several feet in a century. f 
Delta of the Rhone. — We have already alluded to 
the delta formed by the river Rhone in the Lake of 
Geneva; but, after the river issues from that lake, 
its pure waters are again filled with sand and sedi- 
ment by the impetuous Arve, which comes down 
from the highest Alps, bearing along immense quan- 
tities of granitic detritus, swept into the valleys by 
the glaciers of Mont Blanc. Before it enters the 
Mediterranean, whose blue waters it discolours for 
the space of six or seven miles, it receives vast 
quantities of transported matter from the Alps of 
* Lake Superior is the largest body of fresh water in the 
worW, being 1500 miles in circumference, and varying from 80 
to 150 or 200 fathoms in depth, so that its bottom is in some 
parts 600 feet below the level of the Atlantic, and its surface as 
much above it. 
t Lyell's Geology, vol. i., p. 437. 
