48 ANIMAL ENVIRONMENT 



more like those of the southern p'ortion of the coniferous forest (67), 

 and gradually gave way through processes of ecological succession to 

 the species of the present day. Just preceding our period, the mastodon 

 roamed over the site of Chicago. The skeleton of one of these was 

 found in a marsh near Crown Point, Ind., another at Gary, 111. 



IV. EXTENT AND TOPOGRAPHY or THE AREA CONSIDERED' 



The area which we shall consider has its center at a point 18 miles 

 east of Lincoln Park. It extends 67 miles (108. i kilometers) to the east 

 and to the west and 40 miles (64 . 4 kilometers) to the north and 40 miles 

 to the south from this point. Measured from the mouth of the Chicago 

 River it extends 85 miles (137 kilometers) eastward, 49 miles (79 kilo- 

 meters) westward, 38 miles (61 kilometers) southward, and 42 miles 

 (68 kilometers) northward. It is 80 by 134 miles (128.8 by 216 kilo- 

 meters) and contains over 10,700 sq. miles (27,820 sq. kilometers). 



The range of altitude in the Chicago area is not great. The lowest 

 part of the bottom of the lake included in our map is about 80 feet above 

 sea-level. The highest point on the Valparaiso Moraine is 900 feet 

 above sea-level, which gives a range of altitude of 820 feet. The surface 

 of the lake is 581 feet above sea-level. The plain of Lake Chicago is 



1 See frontispiece map. The term "Chicago Area" has been applied to regions 

 varying in extent and direction, according to the points of view and interests of 

 various authors. Chicago biologists have as yet written but little concerning the 

 ecology of areas to the east of Millers, Ind. It becomes necessary to go farther from 

 Chicago every year. The areas in Michigan and Northern Indiana offer the only 

 substitute for those nearer to Chicago which are being so rapidly destroyed. 



The following maps covering the area have been published: 



1. Lake Michigan 



a) U.S. Hydrographic Office, Maps Nos. 1467-75. 



6) U.S. Lake Survey Maps, Custom House Bldg., Detroit, Mich. 



2. Land 



a) County surveyors often publish maps covering particular counties, e.g., 

 LaPorte Co., Ind. 



b) Illinois Internal Improvement Committee, The Water-Way Report, Springfield, 

 1909. 



c) Topographic sheets of the U.S. Geological Survey (prepared for much of the 

 region covered by our map). 



d) The U.S. Land Office has maps of the original land surveys which are said to 

 give roughly the distribution of prairies, forests, and marshes. 



e) Rand McNally & Co. publish maps of all local counties. 

 /) Brown & Windes' (Chicago) map of the Fox Lake Region. 



g) Davis, "Peat" (map of marshes), Ann. Kept. Mich. Geol. Surv., 1906. 



