118 
GEOLOGY 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 are those of prisms whose angles respectively are 120° and 60°. But we often find 
other prisms, whose angles differ materially from the preceding; and from a great number of 
observations, I have found it difficult to determine which system of natural joints predominates. 
The rock to lohich the Utica Slate is equivalent in the Silurian Syste?n. 
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. 
