CHAZY LIMESTONE. 



277 



FIG. 175. 



perhaps the two Sister's Islands. It is only the west border of South Hero that is composed of this rock, 

 but it is well developed, and its fossils are numerous, as at Merriam's Bay. West of Lake Champlain, 

 in New York, this rock is found two miles below the village of Westport, the village of Essex, at Chazy, 

 at Point au Koche, and from thence north to the Canada line. In Vermont its most northern exposure is 

 upon Isle La Motte. 



Upon Plate VIII, Fig. 3, the dark blue color denotes the extent of surface covered by this rock, in Isle 

 La Motte. It occupies the whole of the south part of the Island, while Trenton limestone occupies the 

 north east part. The northwest part is entirely covered with Champlain clays and sand ; even to such an 

 extent that the rocks are not reached in digging wells. This plate shows all the objects of interest upon 

 this island that have been so often referred to above. This plate was reduced from one executed by Prof. 

 Thompson, for the Geological Keport ten years ago. 



The Chazy limestone covers more surface in Vermont than any other of the lower silu- 

 rian limestones, and it seems to be one of the thickest. We have no sections from which 

 to deduce its thickness, except the Snake Mountain section, and one at Larrabee's Point; 

 both of which represent this rock where it is comparatively thin. "VVe should estimate it 

 to be nearly 300 feet thick, where it is most abundantly developed. 



FOSSILS IN CHAZY LIMESTONE. 



The following species of fossils have been obtained from the Chazy limestone of Vermont. They are PTiy- 

 topsis tubulosum (Hall), Rltynclionflla altilis (Hall), R. plena (Hall), 

 Hadurea magna (Hall), Cyrtoceras, , Bucania, 



The genus Phytopsis, a marine plant, is characterized by a great 

 variety in the shapes of the stems ; for the stems may be cylindrical 

 or subcylindrical, straight or flexuous, erect or procumbent, and are 

 generally branched. The branches are diverging and anastomosing ; 

 their structure is cellular, consisting of thin laminae with transverse 

 divisions. Other portions present a reticulated structure. 



The species P. tubulosum has a subcylindrical, sometimes obtusely 

 angular or compressed and branched stem ; the branches inosculate 

 obliquely, or in an ascending direction ; the external covering of the 

 plant is quite thin ; and the center is usually filled with softer ma- 

 terials than the outside, or crystals of some mineral. 



Fig. 175 represents a horizontal or transverse section of some of 

 these stems, showing a concretionary arrangement of the calcareous 

 matter around them, which presents the appearance here represented 

 by weathering. Such examples are common in the Chazy limestone in the northwest part of Ferrisburgh. 



Rliynclionella altilis (Hall.) Rhynconella is a mollusk related to the genus Spirifer. The shell is 

 trigonal, acutely beaked, usually plaited ; dorsal valve elevated in front, depressed at the sides ; ventral 

 valve flattened, or hollowed along the center, hinge plate supporting two slender curved lamellae ; dental 

 plates diverging. 



Fig. 176 represents the dorsal valve of the Rliynclionella altilis. The description of the shell is the 

 following: "Kotund; valves almost equally convex, rapidly attenuating from the middle to the beaks; beak 

 of the dorsal valve small, acute, incurved over the other, which is closely appressed to the dorsal 

 valve; surface of each valve marked by about twenty-four equal rounded radii; eight of the radii 

 are slightly depressed upon the back of the dorsal valve, and much elevated in front, meeting an 

 equal number of slightly elevated radii of the ventral valve." This species resembles some 

 species of this genus higher up in the series. . 



This species is abundant in the rocks at Merriam's Bay in South Hero. Some of the strata 



Phytopsis tubulosum. 



Fio . 176. 



R. altilis 



