ALGONKIAN ROCKS IN VERMONT. A^l 



correlation paper ^ Mr. Walcott represents, probably hypothetic- 

 ally, 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 Archaean.^ 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 evi- 

 dence 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 uncon- 

 formably upon the lower gneisses. The Potsdam is only faintly 

 conglomeratic at the bottom, and the same is true of the quartz- 

 ite 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 

 conglomel^ate 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 



'Correlation Papers, Cambrian; Bulletin U. S. Geological Survey, 1890, PI. II, 

 theoretical cross-section at bottom of page. 



^See Geologic column No. 8, opus. cit. p. 366. 



