60 PROTOZOIC ROCKS. 
tified; it has occasionally green particles of silicate of the protoxide of iron* disse- 
minated through it, and on the whole is rather more compact and darker coloured. 
The only certain methods, however, of determining to which of these formations 
any given rock belongs, is to note the order of superposition; or, still better, to 
determine the nature of the imbedded organic remains, which differ materially in 
the two. | 
No very important subdivisions have, as yet, been observed of the Lower Mag- 
nesian Limestone. The lower beds (F. 2 4) have locally more chert disseminated 
through them; there are also drusic crystals of rose-coloured quartz and calcareous 
spar either disseminated or in veins traversing it. 
An inspection of the diagram of comparative heights and of local sections, appended 
to my own and to Dr. Shumard’s Report, will show not only the relative thickness 
of the Lower Magnesian Limestone, but its elevation and depression at given distances 
up the valley of the Mississippi, at the more important points between Prairie du_ 
Chien and Red Rock. Where the elevation is greatest, the magnesian limestone is, 
as a general rule, thinnest. The variation in thickness must be regarded, however, 
as dependent, more upon the degree of exposure to denuding agencies, than upon 
any great difference in thickness of the original deposit. Where it is greatly 
elevated, a proportional quantity of the mass has been broken down and swept 
away, even down with the general level of the country. 
Its greatest thickness, occurring near the mouth of the Vermilion River, and 
again at the mouth of Wisconsin River, is about two hundred and twenty-five feet. 
SECTION II. 
ITS PALAONTOLOGY. 
Tue Lower Magnesian Limestone of the West has been regarded as nearly, if not 
entirely, barren in fossils. This Survey has, however, developed some variety, 
among which a small species of Lingula and a spiral univalve are the most common. 
Besides these, there are other forms, allied to Hwomphalus and Ophileta of Vanuxem ; 
and Dr. Shumard found, in the same formation, in the district assigned to him for 
examination, on the St. Croix and St. Peter’s Rivers, three species of Trilobites and a 
Terebratula. 
The fossils in these rocks being, for the most part, casts, and then often very 
imperfect, it is difficult, and often impossible, to make out the species. 
For some further particulars on this head, the reader is referred to the Appendix. 
SECTION IIl. 
ITS MINERAL CONTENTS. 
In forming an opinion regarding the mineral-bearing character of a rock, in a 
new country, where no mines are in operation, and where little or nothing has 
* When this mineral occurs, the composition of the rock is similar to that of the green crinoidal 
dolomitic beds, interstratified in Formation 1. 
