l8 NEW YORK STATE MUSEUM 



beneath and the normal stratified red Potsdam sandstone above 

 through a Grenville regoHth constituting a basal Potsdam breccia. 

 This is comparable to the gradations existing today in our southern 

 states from live rock below through various intensities of rock 

 disintegration into residual soils and finally to reworked surface 

 materials above. In the process of Potsdam consolidation these 

 disintegration products were reconverted into solid rock, furnish- 

 ing many puzzling phenomena.^ At many points merely a faint 

 veneer of this material remains coating the Precambrian surfaces, 

 or in glaciated ledges of the quartzite it is sometimes removed 

 from all but the tiny corrosion hollows and pittings of the ancient 

 surface, or is found filling ancient joint cracks so as to simulate a 

 vein. These phenomena are displayed in wonderful variety at the 

 two outliers along the Grass river north of Pyrites (localities 74 

 and 75). In particular at the more northerly of these, in the knoll 

 behind " The Bunch " cottage, a peculiar thin-laminated silicious 

 Grenville rock, weathering with strong reddish color banding, has 

 been uncovered by late erosion (chiefly glacial?) in such a way as 

 to show its face in part plastered over with a mosaic of its own 

 small fragments, which fill also its former joint fissures, and pass 

 upward into a sand-matrix breccia and finally into ordinary red 

 Potsdam. The illustrations (plates 5 and 6) give but an inade- 

 quate idea of this occurrence. Twenty rods west of this, and again 

 on the summit of the (second) knoll 80 rods south, are fine 

 examples of basal breccias filling weather-widened joints in the 

 crystallines. 



Apart from the revelations of the past afforded by these contact 

 deposits, they have a more general interest from the red iron ores 

 they contain.^ These usually occupy slight pre-Potsdam hollows 



^ To this sort of deceptive gradation undoubtedly we may ascribe 

 Brooks's belief that the Grenville and Potsdam were conformable mem- 

 bers of one continuous Cambrian (Taconic) series; Amer. Jour, of Sci., 

 4 (1872O : 22-26. 



' The divergent views of various writers as to the origin of these ores 

 may be gleaned from Emmons 1842 (Second District, p. 97), Vanuxem 

 1842 (Third District, p. 267), T. S. Hunt 1871 (N. Y. State Mus. Rep't 21, 

 p. 88-89), Brooks 1872 (just cited), Smock 1889 (Bulletin 7, p. 10, 43-48), 

 the note by Dr U. S. Grant in Winchell 1893 (21st Minnesota, p. 106-8), 

 Smyth 1894 (13th Rep't N. Y. State Geologist, p. 495-99) and D. H. New- 

 land 1907 (N. Y. State Mus. Bui. 112, p. 38-39). While these writings 

 pertain to the Antwerp-Rossie district, which the writer has not visited, 

 the observations given here, as they were written before any of this litera- 

 ture had been exa-mined, serve as independent corroboration of Smyth's 



