224 SKETCHES OF CREATION. 



er of the transporting currents grew feebler, they bore for- 

 ward only the finer sands and aluminous sediments which 

 repose generally upon the surface of the Southern States. 



The rushing torrents born of the dissolving glacier bus- 

 ied themselves also with the work of excavation. Many 

 an existing valley and river course was determined by the 

 active erosions of this epoch. Many a cut through the 

 rocky ribs of mountains had now to be executed to make 

 way for the escape of imprisoned waters. Many a broad 

 and rock-floored valley became filled, and converted into an 

 alluvial plain, by the rubbish which the torrent deposited 

 in its quieter mood. Many a basin was noAv scooped out 

 which, in the next epoch, became a lake of standing water. 

 The basins of all the larger lakes that have been excavated 

 by erosive action conform in their longitudinal extent to 

 the strike of the underlying formations. A line running 

 through the centres of the great lakes from Chicago to Os- 

 wego, runs approximately along the winding strike of the 

 formations of a certain ao^e. This line shows the configu- 

 ration of the shore of the* continent when those formations 

 were accumulating. It is worthy of particular note that the 

 shore-line was always substantially parallel to the axis of 

 these fresh waters during all paleozoic time. In the Lower 

 Silurian it lay to the north of these waters. During the 

 Devonian it was to the south of the waters. During the 

 Upj^er Silurian it was to the south in the eastern region, 

 and to the north (or northwest) in the western region. We 

 may here seize upon a key to carry with us, and unlock at 

 any time the geological map of the country before the 

 mind's eye (compare Fig. 58). Every one locates instantly 

 and definitely the Niagara Falls and Niagara River. The 

 Niagara limestone was named from the falls, and its out- 

 cropping belt trends east and west at that point. This is 

 the great limestone mass of the Upper Silurian. As in 



