Public Parks. 53 



GEOLOGY. 



The geological structure of the region embracing Chicago and the 

 surrounding country is exceedingly simple. The underlying rock 

 is the Niagara limestone which has a general dip to N. N. E., and 

 consequently sinks deeper as traced lake-ward. This rock is seen at 

 the surface at several points in the city and vicinity. Upon this floor 

 was originally deposited a mass of blue clay, not less than one 

 hundred feet in thickness, but as traced toward the former i"im of the 

 lake it rapidly thins out. This rim is clearly defined in one or more 

 terraces, which are traceable from the head of the lake far into 

 Indiana, but to the west of the city 8^ miles distant, at Harlem, they 

 constitute the divide between the waters of Lake Michigan and the 

 Mississippi. While the lake has receded far below its former level, 

 it has left behind a series of sand ridges, the intervals between which 

 are occupied by ponds, which by reason of the sluggish flow of the 

 water and their sheltered position, have proved favorable to the 

 growth of peat-producing plants, from whose decay have resulted 

 large accumulations of humus or vegetable matter. It is upon this 

 ancient lake-bed that the city is founded. The original surface was 

 diversified by sand banks, most abundant along the lake shore 

 extending occasionally to the depth of 16 feet, by partly filled 

 lagoons, and a vegetable mould (which covers the greater portion 

 of the city), resting sometimes on blue clay, and sometimes on 

 beds of sand and gravel, and occasionally mixed, — the depth of 

 these being governed by their proximity to the Chicago river and its 

 branches. The whole region was originally low, flat and ill-drained. 

 Some of the business blocks are built upon partly filled lagoons. In 

 the soundings made by Mr. Chesbrough, City Engineer, prepara- 

 tory to completing the Tunnel for the Water Works, it was found 

 that the lake-bed was composed of blue clay v\^ith superficial sands 

 above, which shifted in every heavy storm. 



Such a soil must necessarily exercise a decided influence upon 



the health of those living upon it, depending of course whether their 



houses rest upon sand, clay, or humus. Sandy soils absorb and 



retain heat much longer, while the clays and humus are cold, and 



absorb heat slowly. Sand absorbs and retains little water ; clays 



twenty times more ; and humus, or surface soil, fift}- times more than 



sand ; and in this way, to some extent, the relative healthfulness 



of different portions of this city, and even of wards, can be accounted 



for. 



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