208 GEOLOGY OF ILLINOIS. 



siderable portions of this, however, are occupied by coal miners, this being the 

 nearest source of supply for the Chicago coal market. 



Throughout the valley of the DesPlaines,- DuPage and Kankakee rivers, 

 the alluvial deposits constantly remind the observer that this county once 

 bordered the lower end and the outlets of Lake Michigan. The "mounds" 

 along the DesPlaines, which were formerly attributed to the industry of the 

 aboriginal " Mound builders/' are evidently the islands and banks of the old 

 western outlet; while the sandy ridges of the Kankakee valley, apparently 

 identical in structure and in timber overgrowth with those now formed and 

 forming on the shores of the presentlake, tell us of the former existence of either 

 an eastern outlet, by the way of either Deep creek or Salt creek, (in Indiana,) 

 and the Kankakee, or, perhaps more probably, a lake-like expansion of the 

 Kankakee, before it cut down through the heavy bedded Niagara limestone and 

 the underlying shaly calcareous sandstones of the Cincinnati group, which form 

 the high bluff banks of this river along its course through this county. These 

 sand ridges have been traced on the southern side of the Kankakee as far as 

 the mouth of Yellow river, in Starke county, Indiana, and at frequent inter- 

 vals on the north side of it. Further remarks upon this subject will be found 

 in the report upon Kankakee county. 



Among the alluvial deposits of the DesPlaines valley, Mount Joliet claims 

 especial attention, from the fact that it has been made notorious by those early 

 writers who supposed it to be the work of the " Mound-builders," who pre- 

 ceded the Indians in the occupancy of the country, and also because the bed of 

 clay at its base is now made of considerable economical value in the production 

 of brick. This bed is a light drab homogeneous clay, from seven to eight feet 

 thick, of either river or lake origin, and is overlaid by from twenty to thirty 

 feet of a limestone gravel, formed from the outcrop of Niagara limestone, 

 which is continuous for several miles above this point. This is probably only 

 a remnant of a bed which formerly filled the whole valley, and was cut away 

 again by river action before the waters of Lake Michigan were turned from 

 their ancient outlet. 



As subsequent in age to this river and lake alluvium, we may here refer to 

 the large boulders, which are so abundantly distributed over the broad levels 

 which cap the first terrace, in the southwestern part of the county. The ma- 

 jority of them are composed of "green-stone," or "trap," probably from the 

 Lake Superior region, while the remainder furnish representatives of nearly 

 all the varieties of metamorphic rocks. From their position above the black 

 soil, it is evident that they floated to their present position on fields of ice, not 

 long before the river retired to its present lower level. They are especially 

 abundant at points where the surface configuration shows that eddies would be 

 likely to form, which would retain the ice-floes until they had time to melt 



