142 GEOLOGY OF ILLINOIS. 



of this limestone are here of sufficient thickness to afford a tolerable material 

 for foundations and rough walls. Its color is a dark grayish blue, on weathered 

 surfaces, sometimes appearing buff or brown. In this locality, besides the spe- 

 cies already noticed as abundant in other beds, there are found many large 

 Orthocerata, and a great abundance of Ambonychia, together with numerous 

 fragments of Trilobites. The limestone, with some intercalated beds of bluish 

 shale, continues to appear in the bed of the creek for upwards of half a mile 

 above this point, before it finally disappears under the Drift. 



Below Yorkville and Bristol, I observed ledges of this formation continuing, 

 with occasional interruptions, along the banks of the river for nearly three 

 miles, and presenting much the same appearance as those already described, as 

 occurring along the river above Yorkville, but with, perhaps, a greater predomi- 

 nance of shale, as compared with the limestone. Just below Yorkville, the 

 river bank shows from fifteen to twenty feet in vertical exposure of crumbling 

 shale and rock, and an equal amount may be observed at other points below. 

 Some of the thin layers of rotten limeston at this exposure, are extraordinarily 

 rich in certain species of fossils, .chiefly Trilobites, Calymene senaria, Asaphus, 

 etc. The last appearance of these beds, down stream, is at a point not quite three 

 miles below Yorkville, in the southwest quarter of section 36, township 37, 

 range 6. 



The outcrops of this formation which remain to be described in this county, 

 are on the AuSable creek, in township 35, range 8. The intermediate prairie is 

 entirely destitute of outcrops, and, except in the immediate vicinity of the Au- 

 Sable and Fox, no rock in place has been reached by any artificial excavation. 

 The northernmost of the outcrops of this group on the AuSable, occurs in the 

 bed of the creek, very near the center of section 9, and is only visible at low 

 water. The ledge, which is of very limited extent, is of an apparently massive 

 gray crystalline limestone, containing a few characteristic fossils, among which 

 I noticed Rhynchonella capax and one or two other brachiopods. The upper 

 surface is smooth, and covered with faint striae, which have been already noticed 

 in the remarks on the Drift of this county. The next appearance of the rocks 

 of this age, is at the crossing of the county road, on the center of the western 

 line of section 15. Here the bed of the stream at the ford, and for a few rods 

 above and below, is composed of a thinly-bedded, highly fossiliferous, light 

 gray limestone, the beds dipping slightly (8 or 9) to the northeast. The fos- 

 sils here are the same as in the other localities described. About half a mile, 

 in a direction a little east of north, from this place, on the southwest quarter of 

 section 10, limestone, apparently the same as that exposed in the bed of the 

 AuSable, was reached at a depth of ten feet, in digging a well. 



In the bed of the AuSable, near the southern line of section 15, a dark col- 

 ored shale, or shaly limestone, is exposed, which affords many fossils. A little 



