TRENTON LIMESTONE. 



301 



Fio. 215. 



limb, which terminates in long spines ; cephalic shield composed of three prominent convex lobes ; the 

 furrows of the glabella converging towards the axis; thorax with six rings; those of the pygidium variable. 



Trinucleus concentricus (Eaton.) Buckler semi-circular or subcresent form, the posterior angles produced 

 into long, slender, straight spines ; glabella very prominent, finely granulated, produced 

 posteriorly into a short spine ; cheeks prominent, finely granulated ; marginal fillet marked 

 in front by three, four or five rows of deep rounded pores or punctures ; these rows increase 

 by one or two additional ones on the sides of the shield, and towards the lateral posterior 

 angles are often irregularly scattered. 



Fig. 215 represents the cephalic shield of this species, together with the slender spines 

 from the posterior angles. We have seen no specimens of the thorax or caudal shield, in 

 the Trenton limestone, though the cephalic shield is very abundant. 



This species is widely distributed in the Trenton limestone of Vermont. It has been 

 seen at Larrabee's Point in Shoreham, Frost's Landing in Bridport, near Bridport village, 

 near Chimney Point in Addison, in the Trenton limestone at the base of Snake Mountain, in the east 

 part of Ferrisburgh, Me Neil's Point in Charlotte, at Grand Isle, north part of Isle La Motte and at 

 Highgate Springs. A bowlder of this rock was found in Sudbury, containing this species, which must 

 have been transported several miles. 



Geological Position and Equivalency. 



The Trenton limestone lies between the Utica slate and the Chazy limestone. Its 

 position has been noticed upon the Snake Mountain section, but we have another section 

 in Fig. 216, to illustrate its relations to the Utica slate. Its connection with the Chazy 

 limestone is given in Fig. 178. 



Trinucleus 

 concentricua. 



Utica. 



Trenton. 



FIG. 216. 



Coarse limestone 



Vtiea. 



Section in Trenton limestone and Utica slate, at Panton. 



Fig. 216 was originally sketched by Prof. Thompson. It is in the extreme northwest 

 corner of Panton, upon the shore of the lake. The beds of the limestone are quite thick, 

 so much so at times as to be nearly destitute of any marks of stratification. The slate 

 is considerably broken, but it contains distinct graptolites of the Utica slate. 



UTICA SLATE. 



It is difficult to distinguish between the Utica slate, and the shales of the Hudson 

 River Group in Vermont, except by their fossils : hence it is not unlikely that the 

 distinctions between them, which we have given, are not always correct. As we had 

 very few fossils to judge from, we were obliged to separate the two rocks, mostly 

 geographically and stratigraphically. In the main it will be correct. We are not sure 

 that there is sufficient difference between these two rocks in nature, to separate them into 

 distinct groups. If so, the importance of a precise boundary line between them upon 

 the map will be slight. 



Lithological Characters. 



In this respect there is great uniformity. The Utica slates are the continuation of the 

 calcareous shales of the Hudson River Group downwards, until they meet the slaty 

 limestone of the Trenton limestone. There are two principal varieties in Vermont : 



