The Ottawa Naturalist. 9 



THE RENSSELAER GRIT PLATEAU. 



By R. W. Ells, LL.D., F.R.S.C, F.G.S.A. 



A very interesting report has recently been published by Mr. T. 

 Nelson Dale, of the U.S. Geological Survey, styled "The Rensselaer Grit 

 Plateau in New York." His paper is of interest to Canadian geologists 

 since the rocks there discussed form part of the series so carefully studied 

 in the earlier years of the Canadian Survey by Sir William Logan and 

 his assistants in the province of Quebec, and the adjoining stales to the 

 south and described by him under the heading of " The Quebec Group." 

 The area reported on by Mr. Dale was also examined very thoroughly 

 by Sir William Logan, some thirty years ago, and his note books shew 

 many careful measurements and sections of the rock there found which 

 are evidently the extension southward down the valley of the Hudson, 

 of the great series in Quebec which extends continuously from the 

 extremity of the Gaspe Peninsula to the Vermont boundary. The 

 arrangement and description of the strata as given by Mr. Dale, show 

 that the same features are there found as in Quebec; and that the 

 strata are practically the same in character. 



These rocks in Canada consist of green, gray, black and red or 

 purple slates, with heavy beds of gritty sandstones which occasionally 

 pass into fine conglomerates. In the description of the grit and 

 associated elates stated by Mr. Dale on p. 306 of his report, they 

 are said to consist of a dark green exceedingly tough, in some places 

 calcareous, generally thick bedded granular rock in which the quartz 

 grains are apparent and upon closer inspection the feldspar grains also." 

 " This rock is interbedded with strata of purplish or greenish slate 

 (phyl/ite), varying in thickness from a few inches to perhaps a hundred 

 feet .... the thin purple phyllite layers along the west edge of the 

 plateau, contain minute branching annelid trails or fucoidal impres- 

 sions." The conglomerate portion of the grits is thus described : " the 

 pebbles of irregular outline measure from two-tenths to eight-tenths inch 

 in diameter and consist of white, pinkish or blueish quartz, reddish 

 felspar, gneiss, slate and red quartzite and as to relative abundance, 



