8 INTRODUCTION. 



and Blatchley/ and more recently in a bulletin of the Chicago 

 Geographic Society by Salisbury and Alden.' As the map shown 

 in Fig. jg indicates, the topographic and soil areas in the vicinity 

 of Chicago are of three types : morainic deposits, chiefly bowlder 

 clay; the Chicago plain, representing the area covered by the glacial 

 Lake Chicago; and beach or dune sands, connected with the present 

 or former beach lines'. Speaking generally, the Chicago district has 

 three great vegetation types connected with these three soil and 

 topographic types: the mesophytic upland forests of the morainic 

 clays, the hydrophytic lakes and swamps or mesophytic prairies of 

 the Chicago plain, and the xerophytic forests of the dunes and 

 beaches. 



A close analysis shows that the types of plant societies are numerous 

 in each of the three general areas named above. In the morainic 

 areas there are several forest types, as will be shown in the body of the 

 paper, and it is here that the various phases of stream activity with 

 their characteristic plant societies are best seen. The types of vegeta- 

 tion on the plain are fewer, including in the main only the various 

 transitions between ponds, swamps, and prairies. The ancient beach 

 lines present some, but liot much variety, but the dune area of the 

 present beach line presents a rich diversity of plant societies. 



The keynote of this paper is that each particular topographic form 

 has its own peculiar vegetation. This is due to the fact that the soil 

 conditions upon which plants depend are determined by the surface 

 geology and the topography. From the standpoint of the vegetation 

 the topographic relations are more important than the geological. As 

 will be shown later, all kinds of soils may have the same vegetation 

 when placed in similar topographic conditions, whereas the same soil 

 may show many diverse types of vegetation. The topographic condi- 

 tions determine the exposure, tUe presence or absence of drainage, and 

 the humus content of the soil, and are thus of overshadowing im- 

 portance. 



Having related the vegetation largely to topography, we must rec- 

 ognize that topography changes, not in a haphazard manner, but ac- 

 cording to well-defined laws. The processes of erosion ultimately 



'Blatchley. W. S. : The geology of Lake and Porter counties, Indiana. Reprint from 

 tlie Twenty-second Annual Report of the Department of Geolpgy and Natural Resources of 

 Indiana. Indianapolis. 1897. 



'Salisbury, R. D., and Alden, W. C. ; The geography of Chicago and its environs. 

 Chicago. 1899. 



