WALcorr.] GEORGIA TERRANE. 109 



impression that the New York typical Potsdam is about equivalent to 

 the lower portion of the Wisconsin series; and that the Acadian beds 

 of Canada and Vermont, and perhaps the other Atlantic areas, are not 

 appreciably different in age; but the difference in faunae is more the 

 result of conditions upon which life depended than a difference in time. 

 This generalization of Prof. Whitfield placed the * 'Granular Quartz," 

 "Red sandrock" or dolomite, Georgia slate and the Potsdam sandstone 

 of New York and the Upper Mississippi Valley in one geologic horizon, 

 the equivalent of the lower beds of the Mississippi Valley section. 



In 1888 Prof. N. H. Winchell 1 correlated the •« Granular Quartz" of 

 Emmons's Taconic system with the Potsdam sandstone, and mentions 

 its westward exteusion in the Mississippi Valley. In another paper 2 he 

 says that the stratigraphical relations of the "Granular Quartz" and 

 the Potsdam of New York are the same ; that the fossils are cognate 

 with those of the Eed Sandrock and with those that have been found 

 in the Potsdam ; that the Red sandrock overlies the Georgia uncon- 

 formably, and that the stratigraphic relations of the Granular Quartz 

 and the Potsdam sandstone to the granite are the same as those of a 

 Primordial quartzite of the northwest to the granite. 3 



In summing up the opinions of the geological position of the quartzite 

 on the eastern border of the limestone, and between the limestone and 

 the Archean, Prof. J. D. Dana 4 says that he pointed out in 1872 that 

 the quartzite formation in southeastern Dutchess County lay between the 

 limestone and the adjoining Archean, and the same also occurs in other 

 localities ; that he inferred from the position and apparent conforma- 

 bility that its age was Potsdam or that of the lowest beds of the Lower 

 Silurian of the region ; also, that there is a quartzite west of the border of 

 the limestone that overlies the limestone conformably, and is an inde- 

 pendent formation and newer than the limestone. In his stratigraphic 

 conclusions at a later date 5 the Cambrian quartzitesare supposed to be 

 those immediately adjacent to the Archean gneiss and equivalent to 

 the Potsdam sandstone, while those that occurred at a horizon above 

 the limestone are supposed to belong to the Hudson period. 



In a paper on " The Taconic System and its position in stratigraphic 

 geology" Prof. Jules Marcou 6 tabulates the strata referred to the Taconic 

 system in eastern North America. In this, the Saratoga limestone, 

 with Dikellocephalus is the summit of the series, above the Red sand- 

 rock of Vermont and the Potsdam sandstone of Keeseville, New York. 



1 A great Primordial Quartzite. Am. Geol., vol. 1, 1888, pp. 173-178. 



2 The crystalline rocks of Minnesota. General report of progress made in the study of their field 

 relations. Statement of problems yet to be solved. GeoL and Nat. Hist., Survey of Minn., 17th Ann. 

 Rep. for 1888, 1889, pp. 5-74. 



*Op. cit,, pp. 49,50. 



♦On Taconic rocks and stratigraphy, with a geological map of the Taconic region. Am.Jonr. 

 Sci., 3d ser., vol. 29, 1885, p. 209. 



■On Taonic- rocks and stratigraphy, with a geological map of the Taconic regions. Part 2. The 

 middle and north* in part. Am. Jour. Sci., 3d ser., vol. 33, 1887, pp. 408-409. 



6 Am. Acad., Pioc, vol. 20, 1885, p. 224. 



