XX INTRODUCTION. 
Land Office, ought to be reserved from sale, for mineral purposes. Coal and iron, 
in abundance, and also other valuable minerals, have, indeed, been found, and their 
localities carefully determined; but it has not been customary to make mineral 
reservations, on behalf of the United States, except of tracts promising profitable 
veins of lead, of copper, or of one of the precious metals.* 
The coal-measures of Iowa are shallow, much more so than those of the Illinois 
coal-field. They seem attenuated, as towards the margin of an ancient carboniferous 
sea; not averaging more than fifty fathoms in thickness. Of these the productive 
coal-measures are less than a hundred feet thick. The thickest vein of coal detected 
in Iowa, does not exceed from four to five feet; while, in Missouri, some reach the 
thickness of twenty feet and upwards. 
In quality, the coal is, on the whole, inferior to the seams of the Ohio Valley. 
To this, however, some very fair beds form exceptions. 
On the Mankato and its branches, several pieces of lignite were paren up from 
the beds and banks of the streams. Some of this lignite approaches in its character 
to cannel coal; but most of it has a brown colour, and exhibits distinctly the 
ligneous fibre, and other structure of the wood from which it has been derived. 
Diligent search was made to endeavour to trace this mineralized wood to its source, 
and discover the beds where report had located an extensive and valuable coal-field. 
At one point, a fragment was found seventy feet above the level of the river, pro- 
jecting from the drift; but no regular bed could be detected anywhere, even in 
places where sections of the drift were exposed down to the magnesian limestone. 
The conclusion at which those gentlemen who were appointed to investigate this 
matter arrived, was, that the pieces occasionally found throughout the Minnesota 
country, are only isolated fragments disseminated in the drift, but that no regular 
bed exists within the limits of the District. 
The occurrence of strata of brown coal, earthy coal, and bituminous coal, and 
shale, on the west side of Great Bear Lake, as reported by Dr. Richardson, overly- 
ing a vast region of magnesian limestone, like those of Iowa and Wisconsin, ren- 
dered it possible that this lignite might be found in partial beds also on the Man- 
kato; nevertheless, the observations of the sub-corps on that stream do not leave 
any hope of the existence of even such local carbonaceous deposits. On the con- 
trary, it appears most probable that the pieces found have been transported from 
the north along with the drift, perhaps from these very beds on Great Bear Lake, 
or from the cretaceous or supercretaceous lignite formations which were observed by 
Nicollet, and others, off toward the Missouri and Rocky Mountains. 
* A rich vein of lead ore, traversing the Lower Magnesian Limestone, was discovered on the “ Half- 
breed Tract,” south of Lake Pepin; but this being an Indian cession, it was not reported to the Depart- 
ment, for reservation. 
