OF THE NORTHWEST. 



71 



between two and three miles west of Lake St. Croix, one of which is represented to 

 the left of Dr. Shumard's section, No. 1, S. Ten or twelve miles west of Lake St. 

 Croix, this formation is in place down to the beds of the stream. (See Section 5.) 



This sandstone formation appears to be destitute of other minerals foreign to its 

 composition. Its structure is unfavourable for the retention of metallic ores, for 

 reasons previously enlarged upon. 



The narrow belt, coloured a faint yellow tint, stretching along the Wisconsin 

 River, then crossing the Mississippi near Prairie du Chien, and bearing thence, in 

 a northwesterly curve, towards the Upper Iowa, represents the southern exposure 

 of this formation. The same colour, in isolated spots, ranging northeast and south- 

 west between Yellow River and the Upper Iowa, are outliers of this sandstone. 

 Its northern exposure, indicated by the same tint, occurs in the vicinity of St. 

 Paul's, near the mouth of the St. Peter's, and along the Mississippi from Red 

 Rock to the Falls of St. Anthony. It has also outliers east and west of the St. 

 Croix. 



The thickness of F. 2 c, is from forty to ninety feet, its greatest measured thick- 

 ness being at Fort Snelling, near the mouth of the St. Peter's. 



FORMATION III. 

 st. peter's shell limestone. 

 SECTION I. 



ITS LITHOLOGICAL CHARACTER. 



Though of no great thickness, this is not the least important or the least interest- 

 ing of the formations of the Upper Mississippi. 



It consists of limestones disposed in thin, regular layers. These admit of three 

 distinct subdivisions. 



The lowest of these is the purest limestone of this region of country, containing 

 nearly 65 per cent, of carbonate of lime, and 13 per cent, of carbonate of magnesia, 

 and will, without doubt, anord, by burning, better lime than any of the calcareous 

 rocks which we have yet examined north of Lake Pepin. It contains about 13*5 

 per cent, more lime, and nearly 27 per cent, less magnesia, than the Gray Cloud 

 Island rock, — a bed in F. 2, — and. twenty-two parts more lime, and fifteen parts 

 less magnesia, than the shell limestone (the third subdivision), which forms about 

 eleven feet of the upper portion of this formation. 



The middle member of F. 3 A, is more argillaceous than either of the others, 

 and has much the appearance of some hydraulic limestones. The experiments to 

 which it has been subjected, on a small scale, in the laboratory, show, however, 



