118 GEOLOHY OF THE SECOND DISTRICT. 



are recognized in the great quantities of broken slate which are often found at the base of a 

 cliff. There are no beds of roofing slate in this mass : every layer seems disposed to break 

 as above described. From a cursory examination of the system of the natural joints, they do 

 not appear to be confined to certain fixed directions. It is divided into crystalline forms, 

 which arc those of prisms whose angles respectively are 120° and 60°. But we often find 

 other prisms, whose angles diflfer materially from the preceding ; and from a great number of 

 observations, I have found it diiEcull to determine which system of natural joints predominates. 



The rock to ichich the Ulica Shite is equivalent in the Silurian Sijstem. 



Though the fossils which are given as characteristic of the Llandeilo flags have not yet 

 been discovered in the Utica slate, yet I believe it is a member of this formation. This 

 opinion, I confess, is founded upon the similarity of the lithological characters of the two 

 rocks, which I well know is rarely conclusive. There is, however, something in rocks, their 

 structure, mineral veins, arrangement of parts, etc. which may often be employed in identify- 

 ing strata, and which have' proved of great value. So far as these may be relied upon, they 

 go to support the opinion I have expressed. 



Thickness. — The Utica .slate, in the gorges of Lorrain and Rodman, is about seventy-five 

 feet thick : it is, at least, less than one hundred feet. Here we may determine this point 

 with great accuracy, though it may be, and probably is, thicker elsewhere. In all the dis- 

 turbed localities, it is extremely difficult to arrive at even an approximation of its thickness, 

 or scarcely to identify it very satisfactorily, as the disturbances to which it has been subjected 

 have materially altered it. Hence its recognition is difficult, especially when it is associated, 

 or in juxtaposition with shales of the succeeding rock. 



