88 GEOLOGY OF ILLINOIS. 



hands are kept constantly employed blasting them from their adaman- 

 tine foundations. We know of no quarry in Northern Illinois so valu- 

 able as this for railroad purposes and convenience. The top of the hill 

 is covered with a fine, limey, white clay. Gravel and boulders also 

 abound in the neighborhood. About Wiunebago, Argyle, and along 

 south of Harrison, are many light quarries worked into the Galena. In 

 fact, without further particularizing, all that part of the county bounded 

 by our imaginary line circumscribing the Galena, is underlaid, at no 

 great depth, by this famous lead-bearing rock. 



The only fossil found in abundance, is the characteristic Receptaculitoi 

 sulcata. The quarry men and miners speak of it as the "honey comb," 

 "sunflower coral," or "lead fossil." About Kockford specimens are ex- 

 ceedingly numerous, but generally break to pieces before finding their 

 way into the cabinet, on account of the friable nature of the upper strata, 

 in which they are mostly found. Judge MILLER has a specimen almost 

 as round as an apple. This specimen, when we were in Rockford, was 

 borrowed by an enthusiastic geologist, and we did not see it. But few 

 other fossils were found. 



The Blue Limestone. The Blue limestone, or Trenton proper of the 

 the older western geologists, next succeeds the Galena in the descending- 

 order. It is largely developed in the northern and north-western part 

 of the county. It is here a thin-bedded, bluish-gray limestone, calcare- 

 ous, or with a lime base but some of the shaly partings have a clayey 

 base. In the bottom of the deeper quarries, a very blue strata always 

 exists. This is massive, and conchoidal or glassy in fracture, and in the 

 mining region is known as the "glass rock." A line drawn from a point 

 in the western boundary line of the county, some two or three miles 

 north of where the Pecatonica river enters it, along the north edge of 

 the alluvial bottom of this stream to a locality about midway between 

 Shirland and Rockton ; thence east of north to the northern boundary 

 line of the county ; thence west round the county line to the place of 

 beginning, would bound the superficial area underlaid by this deposit, 

 except that the extreme western part occasionally shows beds of passage 

 into the overlying Galena, and except that a considerable patch of the 

 Blue rocks exist in the extreme north-eastern part of the county. 



The first and second railroad cuts, east of Shirland, made by the 

 Western Union Railroad in excavating for their track, afford the best 

 exposures examined for investigating the Blue limestones of the Trenton 

 series. The first is about eight hundred feet long and thirty feet deep, 

 the second is about four hundred and fifty feet long and fifteen feet 

 deep. The rocks are of a whitish-gray color, with conchoidal fracture, 

 becoming darker colored as the lower strata of the quarries are reached. 

 Further west, about Durant, the stone shows a nearer approximation, 



