HUDSON RIVER GROUP. 309 



7. Brecciated Limestones. 



8. Slate filled with veins of calcite, etc. 



This group of rocks lying in contact with the Utica slate, from West Haven, (passing 

 through the center) and the west part of Rutland and Addison Counties, passing at Shel- 

 burne, in Chittenden County, beneath Lake Champlain, and from Burlington to Canada, 

 skirting the east side of Lake Champlain, may be easily divided into two distinct groups, 

 which are distinguished from each other in our Geological Map of the State. The upper 

 group consists almost exclusively of limestone, and the lower group of slates. The first va- 

 riety embraces these limestones, and is of two kinds a pure white or dove-colored limestone, 

 and a dark, impure limestone, often dolomitic. The white variety is composed of the fol- 

 lowing substances : 



Carbonate of lime, . 94.66 



Carbonate of magnesia, . .23 



Alumina and iron, . . 1.09 



Silica, etc., . . . 2.39 



Water and loss, . . .1.63 



100.00 

 A less pure variety is composed of 



Carbonate of lime, . . 70.00 

 Silica and Alumina, . 28.00 

 Iron and magnesia, . . 2.00 



100.00 



These specimens were from Rich's Quarry, in Swanton ; and, particularly the first, give 

 a proper idea of the composition of these beds of limestone. It is sometimes called dove- 

 colored marble, or Swanton marble (not the black marble which is sawed at Swanton.) 

 There is a continuous deposit of this limestone from Canada Line, in Highgate, to St. 

 Albans Bay, gradually thinning out to the south, and appearing several times as insulated 

 beds, south of this bay, but not south of Burlington, its place being supplied by the 

 coarser variety. It is occasionally filled with veins of calcite, as at St. Albans Bay. 



The other beds of limestone corresponding in position to those just described, are some- 

 times quite pure and nearly white, as in Rutland County, or so completely filled with 

 veins of calcite, as to constitute sparry limestone ; or the limestone is thick-bedded, sili- 

 cious, gray or dark brown, and frequently destitute of any evidences of stratification, as 

 is the variety north of Burlington ; or finally, this limestone may be of a fine blue color 

 resembling some of the blue limestones of the upper silurian. Under the range and 

 extent, these beds will be described minutely. 



Clay slate is the characteristic rock of this group. In Vermont, however, upon the 

 western range, it is found chiefly in the south part. Between West Haven and Bridport 

 it is the principal rock. North of Burlington, upon some of the Islands in Lake Cham- 

 plain, and at St. Albans Point, it is also abundant. In the early part of our explorations 

 we were troubled greatly by the impression, fostered by some geologists, that nothing but 

 non-calcareous clay slates or shales ever constituted this formation. The color and hard- 

 ness of clay slate varies from soft green and dark colored clay slate to the green and black 



