154 GEOLOGY OF ILLINOIS. 



county deserves a passing thought. The carbonaceous and bituminous 

 shales found further north, which are there a very marked feature, are 

 here almost entirely wanting, or are at least not distinguishable by or- 

 dinary observation. The stone is coinpacter, and less liable to decay, 

 than that examined in many other localities. It approaches nearer an 

 ordinary limestone in structure and in uses. And altogether it is a 

 valuable deposit, as I shall show in speaking of the economic geology 

 of the county. 



Organic Remains. These are not very abundant. Most of the heavily 

 worked outcrops are barren of fossils. In the ravines cutting the for- 

 mation on the north the Chaetetes petro-politanus, Orthis leatufluxiria, 

 0. occidentalix, 0. lynx, and a small Brachiopod, probably a Leptcvna, 

 are often picked up, weathered out in great perfectness. The spines 

 and shields of a characteristic trilobite, the Asaplms f/ifjas, are not rare, 

 but perfect specimens are seldom found. At Sterling some of the thin 

 layers are exceedingly hard, almost flinty, and are thickly covered with 

 fossils. Conspicuous among these is the iStrophomena alternate^ and 

 many other shells common to this and the Trenton period. 



The G<(lena Limestone. This becomes the surface rock to some extent 

 along the northern and northeastern part of the county. That irregu- 

 larly shaped parallelogram in the latter locality, north of Sugar creek 

 and east of Elkhorn creek, is all underlaid by the Galena limestone. 

 Buffalo creek cuts this piece of land in a diagonal shape. For two or 

 three miles west of Polo this creek runs over and shows exposures of 

 the Blue limestone; but at Sanfordsville, near the line between this 

 and Ogle county, this rock outcrops heavily just below the dam for the 

 saw mill. The outcrop has been worked to the depth of twenty-four 

 feet. The layers are massive, solid, and subcrystaline. At the present 

 time the quarry is not much worked. Following the creek down a few r 

 miles, the next exposure of consequence is at Wilson's mill. Here there 

 is a quarry worked to the depth of about twenty feet. The stone is 

 similar to that at Saufordsville. Other small outcrops may be seen in 

 this locality in some small ravines in a white oak grove of some extent. 



On Elkhorn creek, at Allison's mill, just across the line in Carroll 

 county, there is a worked exposure some eighteen or twenty feet thick. 

 The stone here is of a white cream color, and quite handsome in ap- 

 pearance. From this locality nearly to Bressler's mill, just east of the 

 residence of Dr. PENNINGTON, the Galena is the surface rock. It runs 

 low, however, and soon disappears below the overlying Cincinnati 

 group. 



The central part of that broad stretch of prairie and rough land north 

 of Morrison, and lying between the Mississippi bluffs and Eock creek, 

 is also underlaid by this rock. There is no conspicuous outcrop any- 



