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THE JOURNAL OF GEOLOGY.

palæontological 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 enumerated. 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."[1] Seven years later (1868) C. H. Hitchcock abandoned his theory as to the age of the quartzite,[2] and in a new classification refers it to the Potsdam group. The Talcose conglomerate 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 conglomerate at the base of the Potsdam 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

  1. ↑ Opus. cit. Vol. I., p. 454.
  2. ↑ The Geology of Vermont, Proc. Amer. Asso., 16th meeting, 1868, p. 120.
  3. ↑ Opus. cit. p. 122.
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