I92 TRAXS. 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 jagged promontories have been rounded oft' 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 



