222 GEOLOGY OF ILLINOIS. 



TTi e Hamilton Limestone. 



About a mile and a half below Hampton the upper and more shaly 

 beds of the Hamilton limestone first begin to outcrop along the Missis- 

 sippi river. About Moline still heavier outcrops exist. These are 

 thicker-bedded, are of a brownish color, and are full of fossils. At 

 Bock Island City, and about Camden, it becomes more massive ; the 

 stratification is irregular ; the color a bluish-white or brown upon recent 

 fractures, and the stone hard and tenacious. At Camden the bed of 

 the river is a solid floor of these irregularly-shaped rocks. They are 

 worn smooth by the flow of the heavy, swift-running waters, rushing 

 over them for ages, and stained a mud color by the sediment. It un- 

 derlies all that narrow bottom reaching from Moline to Camden, and 

 attains a thickness in its outcrops of perhaps thirty feet. Rock Island, 

 in the Mississippi river, is a vast pile of this Hamilton limestone, rising 

 in the midst of the stream, overlaid by a thin soil, and covered by a 

 magnificent young foiest. 



The Devonian limestones, as they are developed in this county, may 

 be readily separated on lithological grounds into three divisions. The 

 uppermost division consists of gray and brown limestones, the lower 

 layers rather coarse-grained and completely filled with the shells and 

 corals peculiar to the Hamilton beds. This may be estimated at from 

 twenty-five to thirty feet in thickness, and is well exposed near Anda- 

 lusia, and on the opposite side of the river near New Buffalo. The 

 middle division consists of brown argillaceous limestones and calcareous 

 shales, full of the characteristic shells of this group, and from 30 to 40 

 feet in thickness. This division is well seen between Rock Island and 

 Moline, where a perpendicular face of thirty feet or more in thickness 

 is exposed in the quarries. These shaly limestones are underlaid by a 

 fine-grained, gray or dove-colored, compact limestone, the upper part 

 tolerably massive, but becoming thinner-bedded below. It extends be- 

 low the river level, and is said to have been penetrated in some borings 

 made here, several years since, to the depth of more than a hundred 

 feet. This would make the aggregate thickness of the Devonian lime- 

 stones at this point from one hundred and fifty to one hundred and 

 seventy-five feet. 



Fossils are quite rare in the lower division of this formation, but we 

 found in its upper beds Atrypa reticularis, Alreolites Goldfussiij and a 

 PltilUpsastrea of the same species as that common in the upper division. 

 The shaly limestones of the middle division contain Spirifer pennatus, 

 8. Parryanus, 8. aspera, S. bimesiaUs, 8. wbattenuatu*, 8. inutilis, 8. 

 Jimbriatus, Cyrtia umbonata, Productus subalatus. Stroplwmena demise, 

 8. fragiligj 8. lepida, Orthis loicensis, 0. suborbicularis, 0. Vanuxemi, 



