CHICAGO AND VICINITY^ 



'7 



permit the development of a wealth of trees and shrubs, since a secure 

 foothold is not easily found. However, as the canon broadens out 

 and the slopes become less steep, shrubs and trees come in, though a 

 typical mesophytic forest is rarely seen. The Starved Rock ravines 

 are cut in St. Peters sandstone, those at J.ockport in the Niagara lime- 



FiG. 4. — Side of a canon in the St. Peters sandstone at Starved Rock. Herbaceous shade 

 vegetation on the precipitous slopes. 



stone, yet the vegetation in the two places. is essentially alike; at any 

 rate, the resemblances are greater than the differences. Much has been 

 written on the physical and chemical influences of rocks upon the 

 vegetation. The facts seen here seem to show that the physiographic 

 stage of a region is more important than either. The flora of a youth- 

 ful topography in limestone, so far as the author has observed, more 

 closely resembles the flora of a similar stage in sandstone than a young 

 limestone topography resembles an old limestone topography. A 



