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Page:Popular Science Monthly Volume 9.djvu/565

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ON THE FORMATION OF LAKES.

ures developing by more or less rapid stages along a definite course in the direction of the type of structure selected for our consideration to-day, and, though I am ready to make an act of scientific faith in the existence of such creatures, I confess my imagination fairly baffled in its attempts to depict them, or the road which this particular course of evolution followed. We must wait patiently for more light from paleontology. But we may wait very hopefully. We may do so because the wonderfully rich harvest of fossil remains now being gathered in North America supplies us with good and solid ground for hope.

Already forms have been discovered there so strange that they cannot be satisfactorily grouped in any existing order of mammals—forms such as imagination could hardly have anticipated. We may, then, not unreasonably expect that sooner or later—perhaps very soon—fossils deeply buried in the secondary rocks will come to light, clearly pointing out the line which has been followed in the evolution and development of the only truly flying mammal—the bat.—Popular Science Review.

 

ON THE FORMATION OF LAKES.
By I. C. RUSSELL.

IT was not until the studies of Agassiz, Forbes, and others, among the Alps of Switzerland, had made us acquainted with the character and action of glaciers, that we could at all understand many of the most curious and interesting features connected with the formation of the multitude of lakes with which we are more or less familiar, and which lend so much beauty and grandeur to the scenery of the world.

As some classification is necessary for the understanding of a series of facts, we will arrange lakes under four heads: 1. Those filling glacier-worn rock-basins; 2. Those confined by banks of sand, gravel, bowlders, etc., or, in one word, by moraines; 3. Those formed by a subsidence of their bottoms, or by the elevation of the country surrounding them, commonly by the secular changes of level to which the crust of our globe is subject; 4. Lakes filling basins formed by volcanic action.

1. Lakes which fill rock-basins are such as are confined on all sides by the common rock of the country, so that in some cases a person can walk entirely around them without stepping off the solid rock; and in all cases they would be found to have a rocky rim inclosing them, were the superficial material removed. How such spoon-shaped depressions could be scooped out, was for a long time an enigma which eluded the search of the most painstaking observers.