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ALGONKIAN ROCKS IN VERMONT.
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correlation paper[1] Mr. Walcott represents, probably hypothetically, the quartzite lying unconformably upon Pre-Cambrian (Algonkian) strata. The evidence for a time-break at Clarksburg Mountain in Massachusetts is undoubted, but farther north the relation of the quartzite to the subjacent rocks is much more obscure. As to the age of the subjacent terranes in Rutland County, Mr. Walcott refers them to the Archæan.[2] Since the Olenellus fauna, as determined in Vermont, delimits the base of the Cambrian horizon, all the sedimentary rocks below (adopting the classification of the U. S. Geological Survey) must be referred to the Algonkian. As mentioned above, the quartzite along the border is considered a near-shore deposit, and as such, it is evidence in itself of an approximate subjacent delimitation of the Cambrian sediments. On lithological grounds alone it would be correlated at once with the Potsdam on the eastern border of the Adirondacks, not thirty-five miles west of Wallingford, where the base of the Upper Cambrian is plainly seen resting unconformably upon the lower gneisses. The Potsdam is only faintly conglomeratic at the bottom, and the same is true of the quartzite in Vermont; so that in Vermont, at least, we are apparently without a true basal conglomerate in the Cambrian. The Lower Cambrian lies directly upon granitoid gneiss twenty-five miles south of Wallingford, where the contact is depositional with no conglomerate whatever. These occurrences indicate that we are not obliged to postulate still lower members of the Olenellus horizon on the ground that the base as there shown is not delimited by a conglomerate. In all the localities in Vermont examined by me a reversed dip in the quartzite on the west side of the range has not been observed; in the stratified series just below overturns occur along this line. This may be cited as evidence of discordance at the base of the Olenellus quartzite, as it is extremely unlikely that pronounced overturning could have taken place without involving the quartzite in its folds. That a thick

  1. ↑ Correlation Papers, Cambrian; Bulletin U. S. Geological Survey, 1890, Pl. II, theoretical cross-section at bottom of page.
  2. ↑ See Geologic column No. 8, opus. cit. p. 366.
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