DALE OWEN ON THE WESTERN STATES OF NORTH AMERICA. 441 



Eschara, species undetermined ; Goniatites, and one or two other 

 species ; also some undetermined species of Delthyris. 



In the eight feet of limestone situated between the water-limestone 

 and shale, Dr. Clapp informs me he has found Atrypa prisca, several 

 Crinoidea, Favosites Gothlandica (?), and a small peculiar variety of 

 Favosites polytnorpha. Dr. Clapp, residing at the Falls of the Ohio, 

 and having had an excellent opportunity of studying these and the 

 adjacent rocks in detail, considers the water-limestone as the equi- 

 valent of the Lower Ludlow rocks of Murchison, and also of the 

 Helderberg group (not the Onondaga salt group) of the New York 

 geologists. 



The shale must probably be referred also to the Lower Ludlow, 

 and the New York equivalent is the Marcellus shale. The Encri- 

 nital limestones of Tennessee and the green ferruginous rock of 

 Indiana may correspond with the Aymestry limestone, but of this 

 I am not certain. 



The shale sometimes contains small veins of solid bitumen and 

 imperfect seams of coal, but no coal of any value has ever been 

 discovered associated with it. Pyrites is a very common mineral in 

 these strata. Calcareous nodules also abound in the shale in many 

 localities, often assuming the form of Septaria, which have been 

 mistaken for fossil turtles. 



Next in order we reach a highly interesting group of rocks. It 

 consists almost wholly of compact limestones, lying in thick beds 

 without any interstratified marls or shale, and, for this reason, fre- 

 quently forming cliffs and falls. This bed passes on the Tennessee 

 river, and at some localities in Ohio, into marly or easily disintegra- 

 ting argillaceous limestones. Where it assumes this character in 

 Tennessee, it forms a series of low and sterile hills, known by the 

 name of "glades." These glades or bald knobs, rising from the 

 midst of the dense forest, have a remarkable appearance. 



Siliceous layers and nodules of chert and hornstone are common 

 in this formation, and are often disposed in beds alternating with 

 the limestone, much in the same manner as flints in chalk. The 

 upper beds occasionally become, over a limited area, entirely sili- 

 ceous. Where the surface has suffered from denudation, such spots, 

 more durable than the surrounding material, rise in the form of ir- 

 regular and truncated cones above the general face of the country, 

 and obtain, as being the most conspicuous objects in the north- 

 western landscape, the name of " Mounds." The area of this for- 

 mation is very extensive, as may be seen by reference to the map. 

 It should be remarked, however, that near Lake Erie and Lake Mi- 

 chigan it is covered for a considerable tract with extensive diluvial 

 deposits, so that it seldom emerges to the surface. 



This rock is best developed towards the N.W. In the Du Buque 

 and Mineral Point districts of Iowa and Wisconsin it is the prevail- 

 ing formation, and becomes a true magnesian limestone upwards of 

 500 feet in thickness. An extract from my Report made in 1 840 

 to the General Government will show how closely this magnesian 

 limestone, as it appears in that region, resembles, both in lithological 



