86 GEOLOGY OF ILLINOIS. 



The Trenton Formation. 



The Galena Limestone. Two-thirds of this county is underlaid by this 

 rock. It is a heavy-bedded, yellowish, cream-colored, dolornitic lime- 

 stone, compact, irregular, somewhat crystaline towards the middle and 

 bottom strata, light-colored, porous, crumbling, and full of sand in little 

 cavities toward the top. In some localities the bottom layers pass grad- 

 ually into the blue shaly parts of the Blue division, so that it is difficult 

 to place the line of demarcation between the two. An imaginary line 

 entering the county about the southeast corner of the township of Eoscoe, 

 drawn thence in a south-east course until Rock river was reached ; 

 thence extended round in a slight bend towards the north-west, until 

 within a short distance of the Pecatonica river, at a point about four 

 miles west of its mouth ; thence meandering along the Pecatonica from 

 one to two miles south of the thread of that stream, until the western 

 boundary line of the county was reached; thence starting south and 

 keeping around the boundary line to the place of beginning, and em- 

 bracing about two-thirds of the county, would indicate the superficial 

 extent of this division, to which would have to be added a narrow strip, 

 extending from the village of Pecatonica, up towards and nearly to the 

 north-western corner of the county. The most notable quarries and 

 outcrops within these boundaries were the following. The first heavy 

 outcrop of the Galena limestone exposed on Eock river, after it flows 

 upon the same, is about three miles above Rockford. A high bluff on 

 the north bank of the river presents a bold escarpment, some seventy- 

 five feet in highth. At this place a large quarry is opened. The stones 

 are hard, compact, and subcrystaline, and burn into the very best quick 

 lime. A little steamer, towing a couple of stone boats, makes daily 

 trips in the summer season from this point to the perpetual New York 

 lime-kiln in the city of Rockford, transporting thither the large quanti- 

 ties of stone daily burned into lime at this greedy stone-devouring kiln. 

 Drifting down to the city we find the next heavy outcrops. One mile 

 east of Rockford, along a prairie ridge, there is an exposure about forty 

 feet thick, where a light colored, whitish, friable stone is quarried to a 

 considerable extent. In the timber ridge, about one mile north of the 

 fair grounds, is another, about ninety-six feet in thickness, where the 

 workmen have penetrated entirely through the Galena limestone, and 

 about five feet into the Blue limestone below it. The line of demarca- 

 tion is strongly defined. No brick wall builded upon a stone foundation 

 ever presented a more marked contrast. Three miles below the city, in 

 a bluff on the west bank of the river, is a worked outcrop thirty-five 



