402 THE JOURNAL OF GEOLOGY. 



palaeontological reasons to the Medina, and the hypothesis is 

 advanced that by the removal of silicates by circulating waters 

 metamorphosis of the quartz-rock to the conglomerate has taken 

 place. Reference will be made again to this conglomerate in 

 the following pages. Under the head of Gneiss, rocks of great 

 variation are grouped. Eight principal varieties dependent on 

 accessory minerals such as hornblende and epidote are enumer- 

 ated. The gneiss is represented as a slightly curving band, 

 extending from the Massachusetts line nearly to the north end 

 of the state, gradually narrowing to a point. In the south-eastern 

 part of the state another shorter lense is mapped, but this has 

 not been explored by the writer. The relations of the gneiss to 

 the conglomerate or quartz-rock are not dwelt upon, but many 

 phases are assigned to metamorphosed Lower Silurian rocks, 

 while the probability that even older rocks may be exposed 

 along the anticlinal axis in the range proper, or to the east is 

 regarded as a possibility. A deficiency of feldspar is remarked 

 upon ; because of this peculiarity, according to Hitchcock, 

 Adams called it "Green Mountain Gneiss to distinguish it from 

 true gneiss."^ Seven years later (1868) C. H. Hitchcock 

 abandoned his theory as to the age of the quartzite,^ and in a new 

 classification refers it to the Potsdam group. The Talcose con- 

 glomerate is placed in the " Lauzon " group of the Lower 

 Silurian, while to the Eozoic system the Green Mountain gneiss 

 is assigned. In placing the gneiss in the Eozoic he does not 

 infer that it necessarily is older than the Cambrian or Huronian. 

 Several reasons are enumerated for referring it to this system, the 

 strongest one being the evidence afforded by the occurrence of 

 pebbles in the Talcose comglomerate at the base of the Pots- 

 dam derived from gneissic rocks. An unconformity beneath the 

 Potsdam points to the Eozoic age of the lower rocks. 3 



The suggestion made by Adams (above mentioned) that 

 the Green Mountains are an anticlinal fold, is followed, in 



'Opus. cit. Vol. I., p. 454. 



^The Geology of Vermont, Proc. Amer. Asso., i6th meeting, 1868, p. 120. 



30pus. cit. p. 122. 



