LAKES AND TARNS 131 



with Loch Coruisk in Skye. Loch Duich, on the 

 mainland opposite Skye, is very deep at the end 

 farthest from the sea, and is probably of glacier 

 origin. When a river slowly dissolves a soluble rock 

 such as limestone over which it flows, it may form a 

 sort of lake on its own course. Sometimes under- 

 ground streams dissolve out caverns, the roofs of 

 which fall in, thus forming depressions that serve as 

 the beginnings of lakes. There are other kinds of 

 erosion, but we are only concerned with the general 

 answer to the question : How do lakes arise ? 



The third way in which lakes may be formed is by 

 bending, breaking, or sagging of the earth's crust, so 

 that depressions result. Fractures, displacements, and 

 dislocations of the earth's crust are included by 

 geologists under the comprehensive term "faults," 

 and some of the great lakes of the world such as 

 Lake Nyassa and Lake Tanganyika have arisen in 

 connection with faulted areas. The valley of the 

 Jordan is a strip of country that has sunk down 

 between two parallel faults, and the Dead Sea is a 

 great lake which occupies the deepest part of this 

 depression. It is a rock basin, the result of an in- 

 sinking ; the surface of the water and the rocky floor 

 are respectively 1,292 and 2,592 feet below the level 

 of the Mediterranean. 



The little that we have been able to say in regard to 

 lakes should be read in connection with what has been 

 said in regard to mountains and valleys. The subject 

 should be followed up in such a book as Mr. Philip 

 Lake's "Physical Geography" (Cambridge, 1915). 



Around the edge of the lake there are rooted plants, 

 which sometimes occur in belts. Nearest the shore 



