32 GEOLOGY OF ILLINOIS. 



penetrated, rocks begin to appear in detached pieces, becoming more 

 abundant at a greater depth, until the regular strata of the bed rocks 

 are reached. Now, these pieces are unworn by atmospheric influences ; 

 they lie in horizontal beds, parallel to the strata below ; and are evi- 

 dently the harder portions of the mass, which resisted the influences 

 that changed the rock bed into a clay bed. Nearly all the float mineral, 

 or clay bed mineral now found, is, also, nothing but the ore which has 

 settled down from the decayed rocks, in which it was once held in veins, 

 and mineral-bearing lodes. 



This is also true of the clays covering some of the Niagara, and Cin- 

 cinnati outcrops or bed rocks, for they partake largely of the underlying 

 rocks, from which they have probably been derived. I think a chemical 

 analysis of these clays would show a great similarity or exact identity 

 with the rocks under them. 



Professor WHITNEY'S theory of atmospheric agencies, and no sub- 

 mergence of the lead basin since its upheaval from the Silurian ocean, 

 explains well these unmixed clays, in situ apparently, at the very places 

 where formed ; but it does not explain the great erosion and denudation 

 which has taken place through the productive part of the lead basin ; 

 and is utterly inconsistent with the terraces, loess and drift phenom- 

 ena, plainly manifest in almost every part of this county. If we knew 

 exactly what the drift forces were, and how they acted, we would prob- 

 ably have no difficulty in seeing what influences modified their force in 

 the lead basin. That such a modification did take place in some way, 

 there can be no doubt. 



The blue plastic clays, which lie near the bottom of the drift in other 

 parts of the State, are sparingly developed here, so far as I have been 

 able to observe. The boulder drift, and coarse gravel drift, which lie 

 near the top of the true drift, except the few loose boulders already 

 noticed, are, also, substantially wanting in this region. The yellowish 

 brown clays, red clays, and hard pan, are developed here to a consider- 

 able extent ; but the average depth of the superficial deposits covering 

 the rocks in Jo Daviess county, is a good deal less than in portions of 

 the State farther east and south. The great denudation which took 

 place here, seems to have been followed by transporting agencies, which 

 bore away a large portion of the materials thus disengaged, to other 

 regions. 



The phenomena here observed are probably best explained by sup- 

 posing two epochs, when causes somewhat different in their results were 

 at work. The first was the epoch of erosion and denudation, accom- 

 panied by vast transporting agencies of some kind, probably flowing 

 water or modified drift forces. During this epoch the Niagara limestone 

 was worn down, and the Cincinnati shales suffered disintegration, and 



