CHICAGO AND VICINITT. 



63 



dominate, and especially Quercus coccinea tinctoria. The oak dunes 

 are more common inland and on southern slopes. Probably the oaks 

 follow the pines, but the evidence^on which this is based is not volumi- 

 nous. The pines certainly have a wider range of habitat than the 

 oaks, occurring in wetter and in drier soil, and also in more exposed 

 situations. The mutual relations of the pines and oaks are certainly 



Fig. 87.— Portion of an ancient beach line (Calumet beach) at Summit, showing the char- 

 acteristic oak vegetation, in this case chiefly bur oaks (Quercus macrocar^a). 



interesting and deserve some very careful study. Pine forests prevail 

 on the north or lakeward slopes, and oak forests on the south or in- 

 land slopes. With the pines are other northern evergreen forms, such 

 as Arctostaphylos, while with, the oaks are Opuntia, Euphorbia, and 

 other more southern types. The density of the vegetation on the 

 north side is also in contrast with the sparser and more open vegeta- 

 tion of the south side. The cause for this radical difference on the 

 two slopes is doubtless complex, but it is obvious that the north slope 

 has greater moisture, shadcj and cold, and probably more wind. 

 Which of these is the more important is not certain, but the presence 

 of the northern species seems in favor of cold or wind as the chief 

 factor. 



