Page:Popular Science Monthly Volume 49.djvu/171

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157
HOW THE GREAT LAKES WERE BUILT.

what is bought by her is taxed; while taxes are levied on her product of labor and on the payments for such products. The general result, therefore, has been that the world can buy comparatively little of the Brazilian, and the Brazilian has comparatively little with which to buy of the world.


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HOW THE GREAT LAKES WERE BUILT.

By J. W. SPENCER, Ph. D., F. G. S.

THE framing of the continent was a work of great antiquity Upon that foundation the plains and mountains were slowly built, and out of them the valleys have since been carved. The last touch in the completion of the continent has been the making of the lakes. The work is geologically new, and the knowledge of how the lakes were produced is only a few years old — or about a decade and a half since the students have been seriously attempting to disentangle the complex history of the lakes, and from the maze of disorderly speculation to bring together an orderly assemblage of scattered facts and events. To have partially accomplished this effort, it required tedious waiting for the discovery of connecting links which were not always obtained in their logical order; and it was often necessary to learn how to look for them, and so the footsteps had to be retraced many times before the lost trails were recovered. Many new things have been learned in studying the history of the lakes, but the most striking physical changes have been during the period immediately preceding and reaching into modern times.

High Continental Altitude of Former Times. — In very ancient times the lake district formed a great plateau at a considerable altitude above the sea, with some bordering mountains or high lands. Those ancient plains have since been molded into rolling hills and broad valleys, and the mountains have been worn down to almost plains themselves. When mountain ridges are close upon the sea or adjacent low plains, at so slight an elevation that the streams are all sluggish, then, aided by chemical action, the rains and streams are always washing down the elevated lands, first making ravines and valleys, and then enlarging them into broad plains with low hills, for the level has been reached below which the agents of destruction can scarcely affect the slightly elevated lands, as is illustrated in Figs. 1 and 2.

If the plains were always to remain at low altitudes, increasing slightly in elevation in proceeding landward, above the drainage basins, and with the high lands gone or going, the country would become monotonous without any bold reliefs or the possi-