LAKES. 



309 



retained their saltness where there was no outlet, but have lost it in other cases, by 

 receiving constant supplies of fresh water through rivers, and continually letting off the 

 salt through outlets. Owing to the subsidence of the soil, or the falling in of the roof of 

 a subterranean cavity, in periods of internal convulsion, the superficial waters of the 

 surrounding country have been drained into the hollows thus created, and aqueous expan- 

 sions have been formed. Others have been caused by land avalanches, and the projec- 

 tion of lava currents across streams, occurring within the memory of man ; but the great 

 majority of lakes date their existence from the time when the crust of the globe received 

 its present general outline, and were produced by the waters encountering those inequali- 

 ties of the surface which mark its configuration. A river, impeded in its course by 

 mountainous elevations, either effects its passage by winding about their lateral extre- 

 mities ; or, if enclosed on all sides, it forms a lake, which, rising to a level with some 

 gorge in the interposing barrier, pours its surplus waters through it. A river, also, 

 meeting with an abrupt and broad depression in the plain or valley which it traverses, 

 fills the basin, and forms itself into a lake, the superfluous waters flowing away at the 

 opposite extremity. There is a striking, though in one respect insensible, relation sub- 

 sisting between rivers and considerable lakes. The former visibly feed the latter ; and 

 he latter no less certainly feed the former, though in a manner that is not so apparent to 

 our senses. By a process of evaporation, the lakes are continually giving off a portion of 

 their mass, which rises in the atmosphere in the form of vapour, and again visits the 

 earth in the form of rain, originating the springs and rills, which unite in rivers, flow 

 into the lakes, and replace their waste. There is no machinery of nature more compli- 

 cated, beautiful, nicely adjusted, and benign in its results than this ; for hereby the earth 

 is preserved either from perpetual barrenness, through want of moisture ; or from submer- 

 gence, through the ocean overflowing its present bounds. 



The lakes of primary regions are distinguished by the wild and romantic character of 

 their scenery, rugged precipices and dark forests usually lining their shores, and rocky 



Loch Katrine. 



