192 TRANS. ST. LOUIS ACAD. SCIENCE. 



least resistance, the sides have been eroded, new ravines (gener- 

 ally on the line where two porphyries of different texture came 

 in contact) have been formed, old ones widened and deepened, 

 and I'agged promontories have been rounded off or worn away. 

 Long after the mountains assumed their present general shape, 

 the sea covered the valleys and deposited the Azoic limestones 

 which crop out south of this region ; then the sea probably re- 

 ceded, but again appeared to occupy its old ground, and during 

 this latter occupation deposited the Silurian magnesian lime- 

 stones which now cover the beds of the valleys, and which in 

 places along the mountain sides reach as high as the 300-foot 

 contour line. During the Silurian age, while this deposition was 

 going on, if the sea was not too deep, all these porphyry knolls 

 reared their heads above its surface and formed an archipelago 

 of rocky islands. After many ages, as man counts time, the 

 sea once more receded, not to occupy the ground again, unless it 

 does so at some future time. After the disappearance of the sea, 

 the quieter and less perceptible agencies of nature, but not the less 

 active nor less powerful in their ultimate results, rain and storms, 

 widened and deepened the ravines and valleys, wore away the 

 rocks, and then covered the slopes of the mountains and beds of 

 the valleys with soil from the removed debris. 



