RELATION OF COOK COUNTY GEOLOGY TO 

 ITS MINERALOGY. 



The species found in this vicinity are such as the geologi- 

 cal history of the region would lead us to expect. Precious 

 metals, gems and great variety of species are not to be sought 

 in a region whose geological history has been uneventful. In 

 the region around Chicago, whenever any rock-layers appear 

 in outcrops as at Stony Island and the West Side of Chicago; 

 or in quarries, as at Bridgeport or Rice's; or in other cuttings 

 like the Drainage Canal they consist of a magnesium lime- 

 stone varying from a pure limestone to dolomite and contain- 

 ing greater or less quantities of chert nodules and different 

 iron compounds. These layers belong to the Niagara forma- 

 tion of the Silurian System, and average about three hundred 

 feet in thickness. Immediately under them are the Hudson 

 River shales, about two hundred feet thick; underlain in turn 

 by the Trenton limestone, 250 feet thick; the St. Peter sand- 

 stone, 200 feet thick; aud the Magnesian limestone, 400 feet 

 thick. All of these latter belong to the Ordovician. At no 

 place in this vicinity do the Ordovician formations outcrop. 

 Our knowledge of them is derived from deep-well borings, of 

 which more than a score have been made in the vicinity of 

 Chicago. (Leverett, Water Resources of Illinois, U. S. G. S. 

 R. XVIII. p. 800 for partial list.) Because so deeply buried 

 and at no point appearing, they contribute nothing of impor- 

 tance to the minerals of the region and our source of local 

 minerals is restricted to the Niagara limestone. Nowhere in 

 the region have these strata been penetrated with igneous rock 

 in dikes or laccolites or overflows. Thus, there is an absence 

 of contact phenomena, and consequently of the complex min- 

 eral compounds and valuable metals which usually accompany 

 contact phenomena. 



Neither have the strata been subjected to such pressure or 

 strain as to become metamorphosed. In general, they remain 

 in nearly a horizontal position, dipping but slightly toward the 

 lake. This slight easterly dip was well illustrated in the beds 

 cut through by the Drainage Canal. There are exceptions to 

 this horizontal position. If one were put down in a boat in 



