OF THE NORTHWEST. 51 
stones of this formation (above the Lingula grit, however) other Branchiopoda and 
several forms of Crinoidea, found in peculiar green dolomitic interpolations.* 
In October of the same year, while measuring sections on the Mississippi, between 
the Falls of St. Anthony and the mouth of the Wisconsin, I discovered within a 
few feet of low-water mark, ten miles below Mountain Island, on the west side of 
the Mississippi, laminated grits and siliceo-calcareous layers, charged with an Obolus, 
probably identical with that occurring in the inferior sandstones of Russia; and from 
some of the very lowest of these I collected specimens of a peculiar Trilobite, remark- 
able on account of the spines, with which it is provided, projecting backwards from 
the margin of the pygidium.+ 
Convinced that the formations of Iowa and Wisconsin were destined to divulge 
new facts relative to the paleozoic base in Western America, I caused to be insti- 
tuted, during subsequent surveys in 1848, 1849, and 1850, minute stratigraphical 
and palzontological sections at every favourable locality. The result showed, 
beneath the Lower Magnesian Limestone, at least six different Trilobite beds, sepa- 
rated by from 10 to 150 feet of intervening strata. 
I communicated this fact, in general terms, in my Preliminary Report of October 
11, 1847, and more at large in my Annual Report for 1848, published in the spring 
of 1849. 
* The largest species of Trilobite obtained in this formation, and which I have 
named Dikelocephalus, is figured on Plate I., fig. 1. It occurs a few feet above the 
water level on Lake St. Croix, imbedded in a species of hydraulic limestone (the 
fifth Trilobite bed), near the top of member d, of F. 1.7 
Many of the fossiliferous beds of this formation are densely crowded with 
organic relics; as much so as the most fossiliferous of the blue limestones of Ohio, 
Indiana, and Kentucky. The proportion of genera and species, it is true, is 
not great, but the number of individuals is immense; some slabs are so covered 
with shells, that it would be difficult to place the finger on a spot without touching 
some of them.§ 
If we except the white sandstone, the uppermost bed of F. 1 e, that upon 
which the Lower Magnesian Limestone (F. 2) rests, nothing definite was known, ~ 
up to the period of the present survey, of the nature or character of the underlying 
beds just described; neither had any well-defined organic remains been described 
anywhere beneath the gray and blue fossiliferous beds which form the upper 
* The analysis of a coarse, buff, crystalline variety of these beds, is as follows : 
Moisture, . : Be ‘ + 0-40 
Insoluble earthy matter, 2-74 
Carbonate of lime, . : ; ‘ 48-24 
Carbonate of magnesia, ‘ 42-43 
Protoxide of iron, with a brie of sickuee. : : 6-14 
Loss, . ‘ ‘ ; , 0-05 
100-00 
+ See Tab. 1, Figs. 4 and 5, and Tab. 1, A. Figs. 11, 14, 15. 
{ For a detailed description of this and other fossils occurring in this formation, see Appendix. 
§ See Tab. 1, B. Fig. 1. 
