M^ PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



its structure is sometimes granular, but oftener of a subcrystalline 

 aspect. It occurs in layers from half an inch to two feet thick, in- 

 terposed between beds of marl more or less indurated, the relative 

 proportion of rock and marl varying considerably at different lo- 

 calities. 



The thickness of this group is greatest about the centre of the 

 Ohio valley, where it is estimated at 1000 feet. In the N.W., at 

 Prairie du Chien, it is but 100 feet, and near the Blue Mounds in 

 Wisconsin only a few feet in thickness. 



By referring to the map, it will be observed that two principal 

 anticlinal axes bring this formation to view in the valley of the 

 Ohio. 



Cincinnati in the state of Ohio may be considered the centre of one 

 of these axes, and Nashville, in Tennessee, of the other. 



Visible (according to Troost) in some places in the Sequachee 

 valley, between two spurs of the Cumberland Mountains, in Ten- 

 nessee, it appears again at the base of the Caney Fork ridge, west 

 of this range, and occupies the surface in Davidson county, Ten- 

 nessee. Passing thence beneath superior rocks on the west side of 

 the Cumberland river, it reappears in a N.E. direction in Fayette 

 county, Kentucky, where it again occupies the surface for about 

 100 miles in a northern course, and is once more lost beneath over- 

 lying rocks near the national road both in Ohio and Indiana, not 

 showing itself again on the surface for SOO miles. At Eagle Point, 

 a few miles above Du Buque, on the Upper Mississippi, is the lo- 

 cality where it first emerges above the waters of that stream. 



The superficial area occupied by the blue fossiliferous limestones 

 and marls, at their principal outcrop, is about 10,000 square miles. 



Strata of limestone, supposed by Dr. Troost to be the lowest 

 members of the group in question, are seen, in some places, even 

 east of the Cumberland range, resting unconformably on the up- 

 turned edges of the inferior stratified rocks of the valley of East 

 Tennessee. But though this group emerges to the surface only 

 over a limited area, there is every reason to believe that it is co- 

 extensive with the whole mass of superincumbent strata, for where- 

 ever the streams have cut sufficiently deep, or an axis of elevation 

 has been sufficiently powerful, some members of the blue fossili- 

 ferous limestone come to light. 



No rock can be more fossiliferous than this shell limestone : 

 whole slabs of it may frequently be seen covered with fossil shells 

 and corallines so closely set, that one cannot place the end of the 

 little finger on a spot without touching some of these organic re- 

 mains. It has yielded to the palaeontologist more prolific subjects 

 for contemplation and research than any other group of western 

 rocks, and has enriched our cabinets with numerous interesting 

 specimens of the marine inhabitants of our globe at almost the ear- 

 liest period to which animal remains have been traced. 



Some of the most remarkable and abundant of these are the Iso- 

 telus gigas and Triarthrus Bechi, which have been found, I believe, 

 in the lower seventy-five or eighty feet of this group, sixty feet of 



