4 GEOLOGY OF ILLINOIS. 



If all the soils, clays, sands, and gravels, and other loose 

 materials spread over the face of the country were removed, 

 the probability is that the rocky surface thus displayed, 

 would present valleys of erosion and elevated ridges; but 

 these inequalities would not perhaps be greater than those 

 now appearing on the surface. 



The glacial drift period, and the tremendous forces acting 

 through it, are not well understood by geologists, but they 

 had much to do with the deposition and present arrange- 

 ment of the loose materials, covering the rocks concealed 

 beneath them. 



Clays, sands, and their various mixtures, are originally 

 derived from the decomposition of the primitive rocks. 

 The silent processes of nature, to-day, as in past geological 

 time, are grinding rocks into clays and sands, and re-ce- 

 menting clays and sands into rocks. The affinities of rocky 

 matter can be destroyed by atmospheric and chemical agen- 

 cies, but the elements will still remain. 



There are two theories as to the deposition of the loose 

 materials covering the rocks in this part of the State. One 

 is, that they are derived from the slow decay of the under- 

 lying rocks, leaving the clay in xiln in the exact places 

 where the rocks rotted away. The other is, that the drift 

 forces mingled, mixed, and deposited these loose materials, 

 having gathered them from long distances, and from many 

 and widely separated sources. 



In the part of the State now under consideration, evi- 

 dences of the truth of both these theories can readily be 

 found. In the productive lead region it is now conceded 

 that the drift forces did not act at all, or acted in a modified 

 form. The productive lead rocks are covered by a peculiar 

 reddish clay, derived in large part, I think, from the decay 

 of the upper strata of the rocks. It bears little evidences 

 of mixing or transporting agencies. The top is enriched 

 by A^egetable and atmospheric influences, and has become a 

 thin, poor soil. As it is penetrated, it becomes a reddish 



