OF THE NORTHWEST. 



51 



stones of this formation (above the Lingula grit, however) other Brand hiqpoda 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.f 



Convinced that the formations of Iowa and Wisconsin were destined to divulge 

 new facts relative to the palaeozoic base in Western America, I caused to be insti- 

 tuted, during subsequent surveys in 1848, 1849, and 1850, minute stratigraphical 

 and palaeontological 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 Dikeloceplialus, 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. l.J 



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, ...... 040 



Insoluble earthy matter, . . . . 2-74 



Carbonate of lime, ..... 48-24 



Carbonate of magnesia, .... 42-43 



Protoxide of iron, with a trace of alumina, . 6-14 



Loss, ...... 0-05 



100 00 



f See Tab. 1, Figs. 4 and 5, and Tab. 1, A. Figs. 11, 14, 15. 



X For a detailed description of this and other fossils occurring in this formation, see Appendix. 

 § See Tab. 1, B. Fig. 1. 



