58 PROTOZOIC ROCKS 
somewhat irregular shape, widest in its southern portion, narrowest about the centre 
of the belt, and averaging, throughout, about thirty-five miles in width. Beyond 
these confines, towards the north and east, this formation may still be seen, in 
isolated masses, and sometimes under fantastic forms, protruding through the drift, 
not only as far as the water-shed between Lake Superior and the Mississippi, and 
northwest towards the head waters of that stream, but, as the red sandstones of 
Lake Superior also belong to this formation, showing itself at the promontories of 
Lake Superior, and at the rapids and falls of several of the tributaries of that lake. 
It will further be observed, by inspection of the Geological Chart, that an axis 
of elevation brings this rock to the surface over a narrow belt, crossing the Missis- 
sippi near the mouth of the Upper Iowa, and ranging thence, with a northerly 
curve, towards Mountain Island. 
I have coloured this formation, where it shows itself as a surface rock, chrome 
yellow. Ihave also indicated, by narrow belts of the same colour, along the Mis- 
sissippi, and several of its tributaries, the localities, where, though it does not form 
the immediate surface rock, it yet constitutes a considerable portion of the sections 
of bluffs exposed on either side of these streams. 
SECTION VI. 
ITS LOCAL DETAILS. 
THESE will be found embodied chiefly in the appended Report of Dr. Shumard 
giving detailed. sections on the St. Peter’s, Wisconsin, Upper Mississippi, Barraboo, 
Snake, and Kettle Rivers; local details, regarding the lower members of the same 
formation, will also be found in the Reports of Dr. Norwood and Col. Whittlesey. 
FORMATION II. 
LOWER MAGNESIAN LIMESTONE, 
SECTION I. 
ITS LITHOLOGICAL CHARACTER. 
Tue traveller-who has visited the Upper Mississippi cannot fail to have remarked 
the peculiar outline of hill that bounds the prospect on either side of this pictu- 
resque portion of that majestic river. He must especially have noticed the conspi- 
cuous perpendicular walls of rock, that rise from out the grassy slope, or green 
copse wood, in massive cliffs, and terrace the heights as with interrupted natural 
battlements from the Makoqueta River to Lake Pepin. It is not, however, until 
the geology of the country has been closely inspected, that he is able to discover 
that the rocks composing the hills which present themselves to view below Turkey 
