Page:Popular Science Monthly Volume 9.djvu/571

Rh No description has been given of the newly-discovered lakes of Central 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 portion 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 vegetation 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 volcanic 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 Märjelen-See, which is formed by the glacier of the Aletsch blocking 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 Märjelen-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 existence to the industry of the beavers, who often build their dams in our streams, and sometimes form shallow lakes of considerable extent.