144 GEOLOGY OF ILLINOIS. 



point. The same beds appear in the bed of Little Rock creek, near the 

 quarry, and have been uncovered at one time, half a mile further up the stream^ 

 though not now visible. Fossils were rare at this quarry, and when found 

 were generally ill preserved. A few fragmentary Murchisonia and Pleuroto- 

 maria, only, were obtained. 



Above this place, according to Mr. Post, this limestone may be found in the 

 bed of the Fox, as far as the mouth of Rob Roy creek, in the southwestern 

 quarter of section 35, township 37, range 6, and it appears in a ledge, visible at 

 low water, in the left bank of the river, in the southwestern quarter of sec- 

 tion 34. 



About half a mile below Post's mills, on the right bank of the river, in the 

 northwest quarter of section 3, township 36, range 6, there is another quarry, on 

 the eastern side of a small knoll which rises a few feet above the general sur- 

 face of the bottom land. In this quarry I observed the beds dipping towards 

 all points of the compass, from north around to south by the east, and, from 

 appearances, it seems probable that if the rock was exposed on the western slope 

 of the knoll, it would be found dipping in that direction also. One or two other 

 similar knolls, or slight elevations, occur within a short distance from this, and 

 in one of them, also, the rock has been quarried and presents similar appear- 

 ances. The rock is the same as that worked at Post's mills, a porous, yellowish 

 limestone, full of traces of organisms, but affording very few well preserved 

 fossils. Those collected here were mostly imperfect casts of lllsenus, Pleurotuma- 

 ria, Murchisonia, Subulites, and one or two small fragments of Zaphrentis and 

 Receptaculites. 



Nearly half a mile further down stream, at Black Hawk's Cave, in the eastern 

 part of section 4, the river cuts through a ledge of this limestone, of which 

 about 16 feet in thickness is here exposed. Black Hawk's Cave is a name giv- 

 en to a natural crevice or a small cave in the rock, which formerly extended 

 back into the ledge for some little distance, but which, with several other sim- 

 ilar cavities in this ledge, has now been almost or entirely destroyed by the 

 quarrying of the stone for the construction of a dam across the river at this 

 point. At the northern edge of the exposure, the strata dip down at an angle 

 of five or six degrees. At the other side, on the contrary, the beds break off 

 abruptly. On the right bank of the river, the outcrops continue a few roda 

 further down stream before disappearing entirely. The next appearance of the 

 rock is on the right bank of the river, in the eastern part of section 8, where it 

 is quarried for building purposes. It is here, as in the other localites, a light 

 yellowish, porous limestone, crumbling in some of the uppermost layers, but 

 becoming more solid and better, as a building material, the deeper it is worked. 

 It contains numerous nodules of chert, and casts of fossils, seldom, however, 

 sufficiently perfect to be at once recognizable as to species. 



