OF THE NORTHWEST. 



59 



River, do not belong to the same geological era as those which appear above the 

 mouth of that stream. Nay, so uniform are they in their general aspect, that the 

 miner himself, who has spent the best part of his days in excavating and exploring 

 their recesses, is wont to regard them as identical. So they are, looking only to 

 their chemical composition. Both are limestones, highly magnesian,* in heavy 

 beds, of great compactness and durability ; but they are separated from each other 

 by from one hundred and fifty to two hundred feet of other strata, the upper 

 hundred feet of which teem with peculiar races of fossil forms, constituting a distinct 

 geological epoch, and marking a long lapse of time that has intervened between the 

 period of deposition of these limestones. In my former report, I have designated 

 them the "Lower" and "Upper" Magnesian Limestones of Wisconsin and Iowa. 

 This distinction, as will appear more fully hereafter, is of the first importance in 

 drawing conclusions regarding the mineral value of the country I have been 

 instructed to explore. 



All the conspicuous escarpments of Magnesian Limestone, south of Turkey River, 

 are composed of the Upper of these formations, whilst all those north of Wisconsin 

 River, as far as Lake Pepin, are of the Lower. 



An inspection of hand specimens is in general not sufficient to enable even the 

 geologist to determine from which of these Magnesian Limestone formations it has 

 been taken, so like are the two in general aspect. Viewed on a large scale, 

 however, some distinctive peculiarities can be observed; the Lower Magnesian 

 Limestone has, both in its upper and lower portion, often oolitic bedsf interstra- 



* The proportion of lime, magnesia and insoluble silicates varies somewhat in different varieties, as 

 may be seen by reference to the following table : 



ANALYSIS OF FOUR VARIETIES OF LOWER MAGNESIAN LIMESTONE (FORMATION II.), BY J. G. NORWOOD. 



LOCALITIES. 



Carbonate of 

 lime. 



Carbonate of 

 magnesia. 



Earthy matter, 

 insoluble in 

 chlorohydric 

 acid. 



Precipitate by hydro- 

 sulphuret of ammo- 

 nia in ammoniacal 

 solution, oxide of 

 iron, alumina, and 

 manganese. 



Water and loss. 



Total. 



From the shore of 















Lake Pepin, . . . 



52 



42-2 



4-3 



0-9 



0-G 



100 



On Lake St. Croix, 















below Stillwater, . 



48-3 



36-8 



G-9 



4-3 



3-7 



100 



Gray Cloud Island, . 



51-4 



40-7 



A trace 



4-6 



3-3 



100 



Thirty miles below 















Lake Pepin, . . . 



29-7 



9-7 



00 



0-9 





100 



"j" This rock is composed essentially of one equivalent of carbonate of lime, and one equivalent of 

 carbonate of magnesia ; as, by the following analysis, will be apparent : 



Analysis of buff oolitic bed of Magnesian Limestone, from near Cape Winnebago, intercalated in Forma- 

 tion 1. 



Moisture, . . . . . . 0-50 



Insoluble earthy matter, 

 Carbonate of lime, 

 Carbonate of magnesia, 

 Oxide of iron and alumina, 

 Loss, 



4-84 

 50-93 

 41-13 

 1-74 

 0-86 



100-00 



