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 Hianuaiil, 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. 
n inspection of hand specimens is in general not tattaicnd 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 odlitic beds} imterstra- 
* 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 i), BY J. G. NORWOOD. 
Precipitate by hydro- 
| Earthy matter, | sulphuret of ammo- 
\ Carbonate of Carbonate of insoluble in} nia in ammoniacal 
LOCALITIES. lime. magnesia. chlorohydric | solution, oxide of | Water and loss. Total. 
acid. | pn 5 eae and 
| Mangane 
From the shore of 
Lake Pepin, ... 52 42-2 4:3 0-9 0-6 100. 
On Lake St. Croix, 
below Stillwater, . 48°3 36:8 6-9 4:3 37 100 
Gray Cloud Island,. | 51-4 40-7 A trace 4-6 Hong. 100 
Thirty miles below 
L 29-7 9-7 60 0-9 | 
+ 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 odlitic bed of Magnesian Limestone, from near Cape Winnebago, intercalated in Forma- 
tion 1. 
Moisture, : . : : ; ; 0-50 
Insoluble earthy matter, : 4-84 
Carbonate of lime, ‘ Yee eid : ; : 50-93 
Carbonate of magnesia, ‘ : ' i . 41-13 
Oxide of iron and alumina, i ‘ ‘ ; 1-74 
Loss, : , 3 ' i . . 0-86 
100-00 
