ON THE FORMATION OF LAKES. 



545 



No description has been given of the newly-discovered lakes of Cen- 

 tral Africa, sufficiently accurate to decide to which mode of formation 

 they owe their origin, but, as they are situated in the tropics, it will 

 probably be found that, like Lake Superior, they fill synclinal valleys. 



To this category belong also the truly great lakes which existed 

 in our Western country during Tertiary times, which in the lapse of 

 ages became filled with mud and silt, and now form the greater por- 

 tion of the rich Territories of Nebraska, Dakota, etc. In this region 

 are found, in great numbers, the remains of the huge animals which 

 lived in these ancient lakes, and fed on the luxuriant tropical vegeta- 

 tion that overhung their banks. 



The well-known Salt Lake of Utah is another example of a lake 

 filling an area of depression, and was of far greater extent in past 

 time, as is very plainly shown by the lines of ancient terraces which 

 are so sharply drawn between Ogden and Salt Lake City, nearly a 

 thousand feet above the present level of the lake. 



4. Lakes of the fourth class, such as owe their formation to vol- 

 canic action, are found occupying the bowl-shaped craters of ancient 

 volcanoes, which, as their fires became extinct, furnished convenient 

 reservoirs for the accumulation of water, and in this manner sometimes 

 formed lakes of considerable extent. Streams of lava, also, when 

 they chance to flow in such a manner as to obstruct the drainage of a 

 valley, may serve as a dam, above which the waters soon accumulate 

 and form a lake. 



Besides the kinds of lakes which we have enumerated, there are 

 others, which are of rare occurrence and exceptional in their mode of 

 formation ; such is the beautiful little lake in Switzerland known as 

 the Marjelen-See, which is formed by the glacier of the Aletsch block- 

 ing up the mouth of a tributary valley, and thus forming a wall of ice 

 above which the waters accumulate. This ice-dam breaks away 

 every few years, and allows the complete and rapid drainage of the 

 lake, which often causes great inundations of the valley below. In 

 ancient times a similar ice-dam existed in the valley of Glen Roy, 

 Scotland, as has been shown by Lyell, which, by damming back the 

 waters, formed a lake similar to the Marjelen-See. The waves of this 

 ancient glacial lake chafed and wore its banks, and thus formed 

 terraces at different levels, in the same manner as we often see the 

 little ripples on the pools of water by the wayside cut their soft, 

 muddy banks into terraces, so that, when the water is evaporated by 

 the heat of the sun, their sides are left in a series of little steps ; in 

 the same manner, but on a far grander scale, the terraces were formed 

 which are known as the Parallel Roads of Glen Roy, that have gained 

 a world-wide fame both in science and story. 



In our own country we sometimes find lakes which owe their ex- 

 istence to the industry of the beavers, who often build their dams in 

 our streams, and sometimes form shallow lakes of considerable extent. 



TOL. IX. 35 



