

walcott.] SUMMARY NORTHERN APPALACHIAN. 283 



by the Chazy -Trenton fauna. I have referred them to the Potsdam zone 

 provisionally. 1 If this reference be correct and the rocks in the section 

 are not duplicated by faulting, the Cambrian system has a thickness of 

 19,000 feet in Washington County. 



This section may be greater than the actual thickness of the sedi- 

 ments deposited in this region during Gambriau time. The upper 3,700 

 feet of greenish, hydro-mica shales probably do not belong to the 

 Cambrian, and there may be some reduplication in the section as meas- 

 ured. Eliminating the probable sources of error, 1 think there is at 

 the least from 10,000 to 12,000 feet of strata that may be referred to the 

 Cambrian group. 



A section 35 mile3 to the south, across the extension of this series, 

 exhibits a great development of the shaly portion similar to that in the 

 lower part of the Washington County section, that passes above into 

 the red and purple slate belt. In this section the upper member 

 of the Washington County section is apparently represented by a 

 coarse, greenish sandstone, and in places a fine conglomerate. At the 

 base of this formation the red and green shales are interbedded with 

 the sandstone, the passage from the slates to the sandstone being by 

 intercalation of sand and slate for a distance of several hundred feet, the 

 slate gradually diminishing in volume as the intercalation of the sand- 

 stone increases. The sandstone is estimated to have a thickness of 2,000 

 feet or more, and it corresponds in stratigraphic position, and is on the 

 strike of the 3,700 feet of greenish hydro-mica schistose shales of the 

 Washington County section. The recent work of Mr. T. Nelson Dale 

 proves this series (Berlin grit) to be of Lower Silurian (Ordovician) age, 

 and indicates that the series of green and purple slates beneath may 

 represent the base of the Lower Silurian (Ordovician) and the summit of 

 the Cambrian. 



Our knowledge of the rocks of the Cambrian group in their southern 

 extension into Dutchess County, New York, is obtained from the obser- 

 vations of Prof. Win. B. D wight. The base of the series is the " Gran- 

 ular Quartz" of Stissing Mountain, that had been referred to the 

 Potsdam zone by Profs. W. W. Mather, J. D. Dana, and others. Occur- 

 ring between the gneiss of Stissing Mountain and the superjacent lime- 

 stone carrying the Calciferous fauna, it was natural that this should be 

 correlated with the Potsdam sandstone about the Adirondacks ; and it 

 was not until the fall of 1886 that fossils of the Lower Cambrian or 

 Olenellus zone were found in the quartzite, and in the limestone resting 

 upon the quartzite. 2 Prior to this Prof. D wight had discovered in the 

 more or less arenaceous limestone and argillaceous shaly limestone 

 near Poughkeepsie the presence of the Potsdam or Upper Cambrian 



•Walcott.O. D.: The Taconic System of Eminons and the use of the name Taconic in geologio 

 nomenclature. Am. Jour. Sci., 3d ser., vol. 35, 1888, p. 241. 



a Dwight, W. B. : Primordial rocks of the Wappinger Valley limestones and associate strata. 

 Vassar Brothers Inst. Trans., vol 4, 1887, pp. 206-214. Am. Jour. Sci., 3d ser., vol. 34, 1887, pp, 27-32. 



