INTRODUCTION. XX1 
In further support of this view of the origin of the lignite of the Minnesota coun- 
try, I may add, that every piece, and fragment which the members of the sub-corps 
could find, was collected and brought away, all of which, when put together, and 
weighed, did not exceed ten pounds. 
From the confluence of the War-oju, to the mouth of the Red Wood River, which 
is as far up as the country was explored, different varieties of crystalline rocks, 
alone, make their appearance, varying in height from a few feet to a hundred and 
twenty-five feet. After passing Little Rock, twelve principal exposures are seen 
immediately on the bank of the river in the distance of eighty miles, the intervals 
being covered by alluvium and drift, which hides them from view. The principal 
varieties are granites, and hornblendic rocks, with occasionally syenite. No traces 
of metallic veins worthy of note were observed traversing these formations. In the 
granite, eight miles below the mouth of the Red Wood River, some specular iron 
was found, but only in thin crusts in the joints of the rock. 
The only mineral that promises to be of much value in this region of country, is 
a bed of nodular iron stone, found at a number of localities, both on the Mankato 
and Lesueur Rivers, at the base of the drift, resting either on the magnesian lime- 
stone or sandstone. This argillaceous bed of carbonate and hydrated brown oxide 
of iron, varies from one to three feet in thickness. 
The middle division of the Iowa coal-field affords, at many localities, iron stones 
of various qualities, associated frequently with hydraulic calcareous cement, which 
occurs either in the form of disconnected septaria or regular beds. In the same 
geological position, at many localities, crystallized selenite has been observed, which 
accumulates in quantity high up on the Des Moines; and, finally, a few miles below 
its Lizard Fork, that mineral expands itself into heavy beds of gypsum or plaster 
of Paris, which show themselves on both sides of the river for the distance of about 
three miles, exposed in horizontal beds with a thickness of from twenty to thirty 
feet. 
The iron-stone occurs sometimes in the form of concretionary nodules, some- 
times in continuous bands of several inches in thickness, interstratified in the shales. 
In the chapter embracing the detailed descriptions of the carboniferous rocks of 
Iowa, will be found the analysis of some of this iron ore, together with other more 
precise information regarding it. 
On Soap Creek and its branches, in Davis County, where the middle division of 
the coal series prevails, there are several salt springs, which were tested qualitatively 
on the spot, and found to contain a portion of common salt (chloride of sodium). 
The amount of the precipitated chloride of silver, as well as the taste of the water, 
indicated, however, only a weak brine. By boring, a stronger water might possibly 
be obtained; nevertheless, the shallowness of these coal-measures, the frequent 
