OF THE NORTHWEST. 



69 



a dark shade of purple-blue. Along the valleys of the Wisconsin, St. Peter's, 

 and St. Croix, where the denuding action of the stream has borne away the super- 

 ficial drift and more recent deposits, this limestone has also been laid bare beyond 

 its general confines as a surface rock ; indicating its extension, under the drift, in a 

 corresponding direction, over a considerable extent of country. The narrow belts 

 of blue along these streams indicate its actual surface exposure. 



Near the mouth of St. Peter's River, before reaching the Falls of St. Anthony, 

 this formation is lost to view under drift. 



SECTION VI. 



ITS LOCAL DETAILS. 



These are given in the Report of Dr. Shumard, heretofore alluded to. 



c. st. peter's sandstone. 



Designated on the Sections as Formation 2 C, and superimposed on the Lower 

 Magnesian Limestone, is another sandstone, of less thickness than that previously 

 described. It is the rock which occupies part of the slope between the first and 

 second terrace at Prairie du Chien, and forms the base of the bluffs at the St. Peter's. 

 It constitutes also the lower nineteen feet of the chute at the Falls of St. Anthony. 

 This sandstone, at most of the localities where it has been observed, is remarkable 

 for its whiteness. When examined by the magnifier, it is found to be made up of 

 grains of limpid and colourless quartz. It is even of fairer complexion than the 

 Linn sand, used by the Scotch glass manufacturer, in the preparation of flint glass, 

 and judging, both from its appearance and chemical composition,* I believe that 

 this material would be equally well adapted for that purpose. It was rumoured in 

 the north that it had been tried by some glass manufacturer, but that the result 

 had not fully answered the expectations. If this be the case, it is probable the 

 best quality of sand from that region could not have been selected ; for the St. 

 Peter's country certainly can afford as pure a quartzose sand as that obtained in 

 Missouri, and now I believe extensively used in the glass-houses at Pittsburg. 



The lower beds are neither as pure nor as white as the upper ; at some localities 

 this whole division of Formation 2 differs but little from the ordinary character of 

 the purer sandstones of Formation 1. For this reason, a careful selection becomes 

 necessary, in order to obtain the best material. 



The " New Cave," mentioned by Nicollet, and well known to travellers on the 



* An analysis of this rock gave but two-tenths of one per cent, of foreign matter, which is alumina, 

 with a trace of carbonate of lime. 



Six parts of the purest of this quartzose sandstone, fused in a crucible, with two parts of carbonate of 

 potash and one part of lime, produced a glass of a similar quality and colour to that which was obtained 

 from Linn sand treated in the same way. 



