ISLANDS. 



305 



FIB. 220. 



Rock Dunder. 



Section on Yew Island. 



Rock Dunder and Juniper Island, are remnants of a great belt of Utica slate, that once filled Lake 



Champlain, connecting with Shelburne on the east, and South Hero 

 on the north. Bock Dunder consists of two folds, as shown in 

 Fig. 220 ; a, a, represent two small veins of limestone running through 

 it. The base of the rock is strewed with large bowlders of Winooski 

 limestone and Laurentian gneiss. The surface of the rock has been 

 whitewashed, but the black color of the slate is more conspicuous than 

 the white color of the wash. It is a favorite resort for flies and musqui- 

 toes in the summer; and an alluvial deposit, of considerable thickness, is 

 forming of the skeletons of flies and musquitoes. This deposit would 

 make an excellent fertilizer. 



Yew island presents at its east end a slight irregularity, as shown upon Fig. 221. The strata dip towards 

 one another, forming a synclinal axis. As the sides of the basin were elevated, the bottom of the axis was 

 broken, and the dark lines, a,a, represent the lines of fracture. A portion of the 

 strata has been lifted up, so that it looks like a great wedge, driven into a crevice 

 in the rocks. Below the wedge, there is an opening large enough for a man to 

 crawl into. The water at the level of b,b, fills this opening to the depth of several 

 inches. The sides of the opening are smoothed, like slickensides, by the crowding 

 of the wedge-shaped mass upwards. 



An example of the curvature of Utica slate is given in Fig. 222.. It was 

 sketched by Prof. Adams in 1847. It is at Paine's Point, at the southwest part of North Hero. A point 



adjacent, found by him to abound in trilobites, was christened 

 Trilobite Point; and as the point deserves a name, no more 

 appropriate one can be given. The two points are thirty rods 

 distant, and must have been connected together formerly by an 

 arch. The necessity for this arch is said to be very obvious. 

 The height of the points is about twenty-five feet, while the 

 intervening shore is quite low. We have drawn lines to indi- 

 cate the former extent of the arch. 

 Bange, Extent and Thickness. 



There are two ranges of Utica slate in Vermont. The principal one begins at West 

 Haven, passes north through Benson, Orwell, Shoreham, Bridport, Addison, Waltham, 

 Ferrisburgh, Charlotte, Shelburne, thence under Lake Champlain to Juniper Island, Rock 

 Dunder, Appletree Point, Colchester Point, Law's Island, Yew Island, South Hero, Grand 

 Isle, North Hero, the eastern edge of Isle La Motte, and is last seen in Vermont at Al- 

 burgh. In Canada the range continues towards Montreal, curving westward south of that 

 city, then turning northeasterly again and running down to the Atlantic Ocean, parallel to 

 the Laurentian rocks north of the St. Lawrence River. 



The other range is quite limited. It is found on the west side of an anticlinal in the 

 Trenton and Chazy limestones, in the towns of Addison, Bridport and Panton, and near 

 Split Rock in New York. 



The range commencing at West Haven, belongs to that belt which extends northward 

 from Hudson River. As it enters Vermont, it cannot be distinguished lithologically from 

 the shales that follow, of the Hudson River Group ; and in this region we have discovered 

 no fossils in either. The shales first appear, in ascending the series above the Trenton, 

 about a mile west of West Haven Post Office. Their inclination is much greater than that 

 of the underlying rocks ; hence it is not unlikely that the real dip has disappeared before 

 the obliterating agency of cleavage. 



Utica slate, at North Hero. 



