JEFFERSON COUNTY. 



:]99 



As a wliole, lliis rock is less firm and indurated in this county liian in the Moiiawk valley, 

 or upon Lake Champlain. It appears never to liave been subjected to so much pressure and 

 to such powerful disturbing forces as elsewhere. It presents simply tlic fragile argillaceous 

 slate of authors, without siliceous matter, or beds of shale, as in the succeeding mass. The 

 great differences which may be observed, are those resulting from disintegration, which is 

 rapidly going on where it contains pyrites. 



This mass contains a great abundance of the fossil common to it elsewhere, the Graptolites 

 dentatus (No. 2, p. 279) ; and it also contains the Trocholites ammonius (No. 3), which, 

 however, is rare, so far as my observnlion extends. A smaller and smoother trocholite is more 

 abundant. The species most abundant in this region, are exhibited in the following cut : 



1. Triarthus benkii. 



2. NucuUles scilula. 



3. Cypricanlites sinuata. 



4. Nuculites poststriata. 



o. Avicula. 



6. Lingula rcctilutcralis. 



No. 1. The Triarthus beckii is probably the most constant and characteristic fossil of the 

 rock. I have not observed it in the slaty limestone of the trenton, nor in the shales of Loraine. 

 It is abundant in the gorges at Rodman and Loraine, and upon the route from Adams to 

 Tylerville, wherever the slate is exposed. The head is the part usually found, which, from 

 its peculiar markings, is readily distinguished from the head of the Calymene. 



No. 2. Nuculites scitula, is extremely common immediately below tlie loraine shales, or in 

 the upper part of the utica slate ; and as this part is often absent, the peculiar fossils arc 

 seldom met with. 



No. 4. Nuculites poststriata, is not a common fossil in Jefferson, or at least has not been 

 observed very frequently. 



No. 5. Avicula insueta, belongs to the Mohawk valley. I did not find it in the slate of the 

 Second district. 



No. 6. Lingula rectilateralis, is associated with the Triarthus. 



The inferior part of the slate abounds less in organic bodies than the superior ; in fact, in 

 some districts, as upon Lake Champlain, thick heavy beds appear to be destitute of all of 

 them, except the Fucoides dentatus. Next in frequency of occurrence, is the Triarthus beckii. 

 The remainder of those which I have figured, appear to be confined to the upper part of the 

 rock. 



In assigning the limits to this rock, I have been governed wholly by the presence of the 

 Triarthus ; and as this occurs associated with all the fossils of which I have given figures, I 



