54 GEOLOGY OF ILLINOIS. 



small streams by which this formation is intersected. The Coatsburg coal 

 shaft terminated in this bed at a depth of about two hundred feet below the 

 surface, and we obtained several finely crystallized geodes here in 1860, 

 from the material that had been thrown out of this shaft. This division of 

 the group is about forty feet in thickness, and is well exposed on McGree's 

 creek, and some of its tributaries, and also on Bear creek, and some of the 

 smaller streams in the western part of the county. Locally, this portion of 

 the group becomes quite calcareous, and the beds are then filled with the same 

 species of fossil shells and corals that characterize the lower division. Another 

 species of Archimedes much smaller than that found in the St. Louis group, 

 called the A. Owenana, occurs both in the upper and lower divisions of this 

 group, and is the oldest known form of this interesting genus of fossil 

 Bryozoa. 



The lower division of the Keokuk group, consists mainly of bluish gray 

 limestones in quite regular beds, varying from six inches to two feet in thick- 

 ness, separated by intercalations of buff or blue shale, or marly clay. Towards 

 the base it is very thin bedded and cherty, the flinty material predominating 

 greatly over the calcareous. These beds are well exposed in the upper part of 

 the quarries at Quincy, especially in the northern part of the city, where 

 extensive quarries have been opened in these cherty beds, and also on the 

 small creek at Whipple's mill, where they gradually pass upward into the more 

 regularly bedded limestones above. At Col. Jamieson's place, two miles north- 

 east of Quincy, the regularly bedded limestones of this group, the equivalents 

 of the beds quarried at Nauvoo and Keokuk are exposed, and higher up on the 

 creek above mentioned, and a mile and a half further east, the quarries were 

 opened in this limestone to furnish the foundation stone for Gov. Wood's man- 

 sion, in Quincy. These quarries afforded an evenly bedded, bluish gray, 

 semi-crystalline limestone, in beds from six to twenty inches thick, and furnish- 

 ed large slabs of dimension stone, from the facility with which the rock could 

 be split into the desired form The quarry rock at this point is directly over- 

 laid by the brown shales of the geode bed. 



From Quincy to the north line of the county, this limestone outcrops at 

 various points along the river bluffs, and is well exposed on Bear creek, near 

 the Lima and Quincy road, where it forms a mural cliff from forty to fifty feet 

 in bight. It is also found on all the small streams in the west part of the 

 county as far south as Mill creek, and on both forks of that stream, though 

 not on the main creek. The regularly bedded limestones of this group, are 

 mainly composed of organic matter, and are formed from the calcareous por- 

 tions of the molluscs, crinoids and corals, which existed in such countless 

 numbers in the carboniferous ocean during this period of the earth's history, 

 as to furnish the greater part of the material required to form entire groups of 



