70 GEOLOGY OF ILLINOIS. 



the west bluff of the stream. The outcrop is twenty-five feet thick. The 

 top part is shaly and yellowish ; the bottom becomes heavier and bluer 

 in color. Some of the thin shaly strata are full of a small sized Orthis. 

 These two outcrops are fair representatives of all the others along this 

 stream. Leaving this stream we find no other outcrop in the county. 

 Some indications of underlying Blue limestones prophecy its existence 

 in the south-eastern part of the county, and we have so marked it on 

 the map. 



Some slabs, with fossils similar to those found in the Dixon marble, 

 were picked up ; these, with the fragmentary stems of encrinites, were 

 the only fossils found. A small specimen of "sunflower coral" was 

 found in the Blue limestone, at Eock run railroad bridge, the only one 

 ever found by us in this rock. 



The Buff Limestone. The only place where this, the lower division of 

 the Trenton, is developed in this county, is at Winslow. It is doubtless 

 the underlying rock for a few miles below this place, and on both sides 

 of the Pecatouica river, for this distance. Here it presents very much 

 the appearance of a quarry in the Blue. The top is shaly, thin-bedded, 

 and of a yellowish, chocolate color. At Martin's mill, in Wisconsin, one 

 mile above, the outcrop is much heavier, the bottom layers more massive 

 and very blue. Professor WHITNEY pronounces these exposures out- 

 crops of the Buff, and the fossils seem to indicate that he is correct in 

 this. The lithological character of the quarries would indicate the same 

 thing, but in a less satisfactory manner. On either side of this strip of 

 Buff, and within a short distance of its outcrops, the Galena limestone 

 comes to the surface, so that the latter seems to rest unconformably upon 

 the former, but in following the stream to the northward, a few miles 

 above the mill, the St. Peter's sandstone begins to show its outliers. The 

 quarry at Winslow is worked twenty-three feet deep, and at Martin's 

 mill, thirty-five feet, and at both places it is some ten feet from the bot- 

 tom of the quarries to the surface of the water. Geologically, the 

 locality is one of the most interesting in this part of the State. 



Fossils. We found here many well preserved casts of fossils. Among 

 them the most characteristic were Pleurotomaria subconica ; a large 

 Orthocera, five or six inches in diameter, and some six feet long, with a 

 part of the shell still wanting 5 a Cypricardites niota f Oncoceras pandion ; 

 some two species of TeUinomya : and some other fossils, which will be 

 mentioned in our catalogue of the fossils of this part of the State, in 

 the prefatory chapter to these county reports. 



