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 parts 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 Centra] 

 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 oi 

 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 ol 

 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 foi 

 the space of six or seven miles, it receives vast 

 quantities of transported matter from the Alps oi 



* Lake Superior is the largest body of fresh water in thf 

 world, being 1500 miles in circumference, and varying from 8(j 

 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 Ly ell's Geology, vol. L, p. 437. 



