280 BIRDSEYE LIMESTONE. 



so homogeneous that it resembles flint brittle, breaking with a conchoidal fracture, and 

 the surface of it often presents numerous crystalline points, whence its name. 



The birdseye limestone is very rarely found in Vermont. Prof. Adams collected 

 specimens of it with its characteristic fossils, and the present corps of surveyers found 

 specimens of it at South Hero, Ferrisburgh, and at Crown point. We did not discover it 

 at Isle La Motte, although the distinctions are so clearly drawn there between all the other 

 limestones of the lower silurian. 



The characteristic fossil is the Phytopsis tubulosum (Hall), which we have already described is one of 

 its obscurer forms. The crystalline points which give the name to the rock, are produced in the interior of 

 this fucoid ; and they sometimes remain even after every other trace of the vegetable has disappeared. 

 Besides the Phytopsis tubulosum, we have obtained but two species of fossils in this rock in Vermont, and 

 these are too indefinite to allow the determination of anything but their generic character. They belong to 

 the genera Maclurea and Orthoceras, and are from the northwest corner of Benson. The Phytopsis cel- 

 lulosum (Hall), has also been found in Vermont. " Its stems are subcylindrical or compressed, diverging 

 from a center or root, procumbent or ascending, branching ; its branches are irregularly anastomosing, form- 

 ing a close strong net work ; the crust or covering is thin ; its substance cellular ; cells oblong, quadrangu- 

 lar or stellate." The Phytopsis tubulosum and P. cellulosum, when crystallized in a compact rock, cannot 

 be distinguished from each other. In perfect specimens, they may be distinguished by their internal struc- 

 ture and mode of growth. The former species is the more common one in the Mohawk Valley of New 

 York, and the latter species is the more common one in the northern part of the Champlain Valley. 



There are two species of trilobites in the birdseye limestone on Isle La Motte, Calymene multicosta 

 (Hall), and Illcenus crassicauda (Dalm.) The genuine relations of the first species are not fully determined. 

 The thorax and caudal shield have, at least, twenty-five articulations. This species has been discovered 

 only at Isle La Motte, and though obscure, was determined to be evidently distinct from anything else in 

 the older strata of New York. Prof. Hall figured it in the hope of calling attention to the occurrence of 

 such a species, particularly to the explorers of Vermont Geology. 



Illcenus. Head semi-elliptical or globose, usually wider than long, or transverse; eyes distant and lateral; 

 rings of the thorax simple, eight or ten ; pygidium very convex, and trilobation rudimentary. 



Illcenus crassicauda (Dalm.) The following is the description of the species /. crassicauda. " Oval with 

 the longer axis more or less extended, convex ; buckler large, convex, rotund ; posterior extremities obscure ; 

 eyes prominent, distant from the axis ; maxillary shield small ; thorax distinctly three-lobed, the divisions 

 continuing a short distance into both the cephalic and caudal shields ; articulations ten, smooth, slender, 

 those of the lateral lobes extended ; caudal shield large, semi-circular, convex, having the rudiment "of a 

 central lobe ; entire surface smooth, or with fine curving, subimbricating lamellose stride." 



The locality of this species is the same with the preceding. 



BLACK EIVER LIMESTONE, OR ISLE LA MOTTE MARBLE. 



The New York Geological Reports have described one of the best developments of this 

 rock in Vermont, so that we do not deem it necessary to dwell upon it here. It is the 

 black marble of Isle La Motte, which is better developed in Vermont than New York. 

 It is at this locality a black, compact, finely granular marble, and is a pure carbonate of 

 lime. It forms at Hill's north quarry but one bed, from twelve to fourteen feet in thick- 

 ness. Several well defined vertical joints cut through the marble, and thus the marble 

 can be easily obtained of almost any desired dimensions. When a part of the bed has 

 been removed, the joints make an almost insurmountable wall of the remainder. 



