8 GEOLOGY OF ILLINOIS. 



sional outcrops at the foot of the bluffs, for about six or eight miles, when the 

 color of the beds change to a bluish gray, very much like the beds in the 

 vicinity of Hamburg, and from thence northward, only a few feet in thickness of 

 the upper part of this formation is seen. A half mile above Hardin, there is about 

 twenty-five feet of the upper part of this formation exposed above the level of 

 the river at low water, consisting of rough, irregular bedded, bluish gray lime, 

 stones. From this point northward, to the small creek jvhich empties into the 

 Illinois about three-quarters of a mile below Farrowtown, we find occasional 

 outcrops of the upper part of this limestone, and on this creek which is the 

 most northerly outcrop known on this side of the county, there is about ten or 

 twelve feet of the upper part of this group exposed, consisting of even bedded, 

 fine grained limestones, that may be seen for a distance of two or three hun- 

 dred yards to the westward of the road. But few fossils were obtained from 

 this formation in this county, though the beds in the vicinity of Monterey 

 seemed to be quite as fossiliferous as this rock usually is in this portion of the 

 State, and when the quarries here are worked to any considerable extent, as 

 they now are at Grafton, they will, no doubt, afford a good many interesting 

 forms of organic life. 



Hamilton Limestone. This is the only division of the Devonian system, that 

 has been identified in this county, and consists of from six to twelve feet of 

 brownish gray limestones, that are usually very hard and silicious, and some- 

 times pass into a coarse quartzose sandstone. At the most northerly outcrops 

 of the Niagara limestone in this county, there seems to have been no develop- 

 ment of the Hamilton beds, and the Upper Silurian limestones are immediately 

 overlaid by the shales and limestones of the Kinderhook group. 



-On the west side of the county, the first exposure of this limestone met with, 

 below the north line of the county, was on the northeast quarter of section 20, 

 township 8 south, range 3 west, where a stratum of white sandstone, about a 

 foot in thickness, was found resting upon the Niagara limestones. No fossils 

 were obtained from the sandstone at this locality, but further south a similar 

 sandstone abounds in the characteristic fossils of this group, leaving no doubt 

 as to the age of these arenaceous strata. In the bed of the small creek, which 

 enters Bay creek about five miles above Hamburg, there is about six feet in 

 thickness of coarse brownish gray limestone exposed, filled with characteristic De- 

 vonian fossils, among which are two or three species of Spirifers, Atrypa recticu- 

 laris, Orthis lowcnsis, and several of the common corals of this group, among 

 which were large masses of a coral which has usually been referred to the genus 

 Acervularia, and has been called A. Davidsoni* At an old mill, a short dis- 

 tance below this point, these thin bedded limestones were eight feet in thick- 



*0n referring this coral to Dr. Rominger, of Ann Arbor, Michigan, one of our best authori- 

 ties on fossil corals, he has pronounced it a true Cyathophyttum. 



