174 GEOLOGY OF ILLINOIS. 



six feet below the surface at Tiskilwa, and nearly two hundred and six- 

 teen below the surface at Princeton. The latter well, however, was 

 commenced on the upland prairie ; the other in the bottom of a deep ra- 

 vine, with the Tiskilwa coal mines in the sides of the bluffs above the level 

 of its mouth. 



Outcrops of the Coal Rocks. Natural outcrops of the rocky forma- 

 tions are very rare in this county. No county yet examined by me 

 shows so few. 



Coal valley is a little valley coming in from among the Big- Bureau 

 bluffs from the south just above Tiskilwa. A ravine about two miles 

 in length comes into this little valley from the west, about one mile and 

 a half 'from its mouth. This ravine is known by the name of Eocky Run. 

 A little stream tumbles down among the rocks and boulders with a very 

 rapid descent. The only stone quarry in this part of the county is in 

 the bed of this tumbling stream. Huge masses of a hard, sub-crysta- 

 lirie, quartzose sandstone block the ravine in places. Considerable stone 

 for foundations, and other economical uses, have been quarried and 

 blasted from these masses. They are not in situ, and are not the natu- 

 ral outcrop of the formation to which they belong, but seem to be out- 

 liers detached from the parent strata, which are undoubtedly in situ 

 under the clay bluffs on either side. These large outliers are somewhat 

 water- worn. Among them are many large erratic boulders of granite, 

 trap, hornblende, and quartz. One or two of the granite boulders are 

 very micaceous. A coal seam, one foot thick, outcrops in the midst of 

 these large detached stones, underlaid by a heavy bed of blue plastic 

 soapstone. A section made in this little run showed the following ap- 

 proximate figures : 



Section half way up Rocky Run. 



1. Bluff clays, yellow and buff. 40 to 60 feet. 



2. Gravelly clay, full of boulders and blocks of sandstone 15 ' 



3. Coal, stained with iron 1 ' 



4. Blue, silvery, unctious soapstone 10 



The bottom of this soapstone is still a good many feet above the 

 mouth of the artesian oil-well, whose section has already been given. 

 I have called this outcrop a sandstone ; but some of its outliers, espe- 

 cially farther up the ravine towards the barn of Mr. Whiting, present 

 the characteristics of a limestone. The rock is entirely unfossiliferous 

 so far as I could see. A good specimen of Lepidodendron was found in 

 this ravine a few years ago, according to local report. In geological 

 interest and picturesque scenery this little run is an interesting spot. 

 I shall speak of its coal seam hereafter. 



The next outcrops worthy of attention were fouud in following the 

 Illinois river bluffs from Bureau to Peru. About two and a half miles 

 east of the village of Trenton, along the base of the bluffs, which rise 



