24 GEOLOGY OF ILLINOIS. 



dark, hard, iron-looking consistence. These weather out 

 in places, giving the appearance of pictured rocks. They 

 are caused by thin crusts of the sandstone, having become 

 impregnated with a strong solution of the oxyd of iron at 

 various times, while the rock was in course of deposition. 

 Many of these broken- crusts resembles pieces of old cast- 

 iron pots. 



Some of them are beautifully marked with what I have 

 been accustomed to consider wave or ripple marks. Prof. 

 WHITNEY could find no evidences of the action of water, 

 in his examinations of this rock further south. I have 

 lately come to think, however, that these beautiful mark- 

 ings, instead of being ripple marks, may be wind marks. 

 In examining some sand blows and dunes lately, I found 

 them about their bases in sheltered positions, marked with 

 the same wave-like etchings. 



However that may be, it is evident that some unusual 

 conditions must have existed when the St. Peter's sand- 

 stone was deposited. It underlies the whole lead basin, 

 and outcrops heavily for a considerable distance around it, 

 reaching on the south into Missouri ; but nothing like it, 

 so far as I know, is found any where else on the continent. 

 Its origin is not well understood. The conditions of its 

 deposition are involved in mystery. 



This is the lowest rock in the series which outcrops in 

 North- western Illinois. The floor of Rock river, I think, 

 is made up of the top of the Calciferous sandstone, at sev- 

 eral places between Oregon and Grand DeTour, but the 

 formation outcrops 110 where at the surface, so far as I am 

 informed. 



The economical geology of this region will be found fully 

 treated of in the detailed county reports, to which this 

 chapter is but an introduction. The ores of lead and zinc, 

 clays, sands, rock for lime, and for building purposes, peat, 

 and many ores and minerals of scientific interest, abound, 

 and are fully described in their appropriate places. 



