HOW THE GREAT LAKES WERE BUILT. 157 



what is bought by her is taxed ; while taxes are levied on her prod- 

 uct of labor and on the payments for such products. The gen- 

 eral result, therefore, has been that the world can buy compara- 

 tively little of the Brazilian, and the Brazilian has comparatively 

 little with which to buy of the world. 



HOW THE GREAT LAKES WERE BUILT. 



By J. W. SPENCEK, 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 mak- 

 ing of the lakes. The work is geologically new, and the knowl- 

 edge 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 or- 

 derly assemblage of scattered facts and events. To have partially 

 accomplished this effort, it required tedious waiting for the dis- 

 covery 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 im- 

 mediately preceding and reaching into modern times. 



High Continental Altitude op Former Times. In very 

 ancient times the lake district formed a great plateau at a consid- 

 erable 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 ele- 

 vation that the streams are all sluggish, then, aided by chemical 

 action, the rains and streams are always washing down the ele- 

 vated 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, increas- 

 ing slightly in elevation in proceeding landward, above the drain- 

 age basins, and with the high lands gone or going, the country 

 would become monotonous without any bold reliefs or the possi- 



