THE BOTANISTS OF PHILADELPHIA. 3 



clays, comjDosing by far the greater extent of the so-called 

 West Jersey tertiary formation, with the exception of a 

 narrow band of the cretaceous green sand and marl Ijeds, 

 potter's clay, fire sands and clay, which parallel its course 

 with the Delaware River, extending in a north-east direc- 

 tion to Raritan Bay. The western water-shed is traversed 

 by streams, which, rising in the marl district and yellow- 

 gravel region of the interior, flow into the Delaware, being 

 affected in their lower reaches by tide-water. The eastern 

 water-shed is intersected by several important streams, 

 such as Mullica, Great Egg Harbor and Toms Rivers. 

 These rivers mainly take their rise in cedar swamps and 

 sphagnum bogs for which the region is noted. North of 

 the marls, as we approach the mountians, a region in which 

 red shale mainly predominates, is entered upon. West of 

 the river, an undulating plain along the river front rises 

 gradually to the older paleozoic hills, which reach an eleva- 

 tion of two hundred feet or more. Back of these, as the 

 Blue Ridge is approached, the country becomes more 

 undulating and broken by numerous hills of various 

 geological formations. 



Enough has been said by way of introduction to show 

 that these topographic, hydrographic and geologic features 

 have an important bearing on soil formation, and conse- 

 quently on plant life and distribution. We find that each 

 topographic, hydrographic and geologic district has some 

 plants peculiar to it. Each of the plant communities, into 

 which the flora of a district as large as Philadelphia can be 

 divided, can be distinguished by the component plants, 

 which, together with their collective features, give character 

 to the vegetation of the particular geological, topographical 



