10 



Utica Slate and 



be termed beds of passage to the Utica slate, and have not, to the 

 writer's knowledge, been found lower in the formation. During 

 eight years of work in the Trenton limestone I have never seen a 

 fragment of this trilobite below the upper shaly passage beds. It 

 has a distinct geological horizon, and its presence indicates a 

 parallelism of formations as will be shown by the facts presented in 

 this paper. The presence of the Utica slate horizon does not rest 

 solely on the presence of this trilobite as there is both a palseontologi- 

 cal and lithological break in the series, at this horizon, throughout the 

 Appalachian basin with the exception it may be at the extreme 

 north-western and south-eastern outcrops. 



In South-western Illinois, the formation resting on the Trenton 

 limestone is described by Professor A. H. Worthern ( Geol. of III., iii, 

 p. 27, 1868 j, as a sandstone, which he calls the "Thebes sandstone 

 and shale." He says : 



"This formation, which underlies the limestone above described 

 (dark blue compact limestone) is well exposed in the A^icinity of 

 Thebes. ***** The lower portion of it only is a true sandstone, 

 and is about thirty feet in thickness, and passes upward into a sandy 

 shale of a dark brown color. ***** x half mile below Thebes 

 we found a yellowish brown shale, apparently not above five or six 

 feet in thickness, that evidently formed the base of this group.. It 

 Avas filled with fragments of Trilohites, apparently belonging to the 

 Asax)hus canalis, which, with a Lingula found in the upper shale 

 immediately below the limestone, are the only fossils it has afforded. 

 ***** Some of the sandstone layers are from two to three feet 

 in thickness and well adapted for building purposes." 



In the valleys of Virginia, Eastern Tennessee, and Northern Ala- 

 bama, the black carbonaceous shales of the Utica slate are replaced 

 by the lighter colored marls and shales of the same geological 

 horizon. 



Professor James Safford in the Geology of Tennessee, pp. 228 — 

 273, 1869, describes the Trenton and the Nashville rocks in East 

 Tennessee as follows : 



" They are, first, a stratum of blue limestone, more or less argilla- 

 ceous, from 200 to 600 feet thick, then, above this, a great body of 

 sky-blue calcareous and often sandy shales. * * * * In going to the 

 north-west the shales become more and more calcareous approaching 

 the condition of the same strata present in Middle Tennessee. 



" At the base of the sky-blue calcareous shales in the eastern 

 border counties there is a fine dark or black shale, becoming in places 

 100 or 150 feet thick. It abounds, very generally, in graptolites. The 

 graptolites are not confined to this lower stratum, they run up into 

 the main body of the shale and are found at numerous localities. 



" In the basin of Central Tennessee the rocks are mainly blue 

 limestones throughout. The entire series is about 1000 feet thick 



