ADAMS COUNTY. 55 



limestone strata. All these animals secrete the carbonate of lime to form the 

 habitations in which they live, and the solid integuments of their various parts, 

 and these calcareous fragments, cemented together by the chemical precipitation 

 of the mineral matters held in solution by the waters of the ocean, now consti- 

 tute many of the limestones and marbles, out of which our cities are built, 

 and which enter so largely, under various forms, into the economic uses of 

 human life. The alternations of limestone with seams of clay or shale, indi- 

 cate the changing conditions that prevailed in the ocean at this time, as these 

 clay seams are formed by the muddy sediments that at various times were intro- 

 duced by currents or other causes into the ocean, which, settling to the bottom, 

 formed the shaly sedimentary strata by which the limestones are separated. 

 The characteristic fossils of this group occur almost everywhere, that the rock 

 is exposed. In the debris of the old quarries northeast of Quincy we found 

 Archimedes Owenana, Agaricocrinus Americanus, Actinocrinus pernodosus, A. 

 l>iturbinatus, Spirifer Keokuk, Productus punctatus, and Zaphrentis dalii. In 

 the quarries at Quincy we obtained Aviculopecten amplus, Spirifer striding, and 

 Productus semireticulatus, from the cherty beds at the base of the group. 



Burlington Limestone. This formation differs but little in its lithological 

 characters from the lower portion of the Keokuk limestone, but it is usually of 

 a lighter gray color, and contains intercalated beds of buff or brown limestone, 

 while the bands of argillaceous shale, which separate the beds in the Keokuk 

 group, are not seen in this. There is, however, one band of green clay, or 

 clay shale, from one to six inches in thickness, intercalated in the beds at 

 Quincy about midway from the bottom to the top of the exposure at the lower 

 end of the city, where the beds are well exposed. At the quarries in the 

 upper layers of the limestone, opposite the steamboat landing, the cherty beds 

 belonging to the Keokuk group are quarried, but in the lower part of the city, 

 the underlying limestones are well exposed, and are extensively quarried to 

 supply the demand for building stone, and for burning into lime. The rock is 

 tolerably even bedded, and affords some layers two feet or more in thickness 

 which, when free from chert, may be cut with facility, and forms an excellent 

 building stone. 



The following is a section of the rocks exposed in the bluffs in the lower 

 part of the City of Quincy : 



Loess capping the bluff 62 



Thin bedded, cherty limestone, (Keokuk.) 13 



Light gray limestone, (Burlington.) 12 



Band of green shaly clay, (Burlington.) 4 in 



Buff and light gray limestones, (Burlington.) 36 



