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LETTERS

OF

MR. O. H. ST. JOHN

ASSISTANT STATE GEOLOGIST.

SURFACE GEOLOGY OF THE RACCOON RIVER REGION.

During the past season, through the kindness of the Director of the Geological Survey, the writer was permitted to spend a considerable portion of the autumn in the country traversed by the Raccoon rivers, with the view of studying the stratigraphical geology of that section of the State. The counties of Polk, Dallas, Guthrie, Greene and Carroll held out important inducements to that end, and the result of the season's work was satisfactory in a high degree. The counties lying to the southward and bordering upon those above mentioned, had been already examined, and their geological structure made known through the labors of Dr. White during the previous season. But heretofore little, I may say nothing, was known of the character of the strata in the country drained by the Raccoon rivers, save that they were coal - bearing. Their relations to the well-developed upper coal - measures found along the North, Middle and South rivers, in Madison and adjoining counties, and to the lower coal measures which are exposed on the Des Moines, remained to be determined. Nor was the nature of the coal - beds themselves any better known beyond the localities where they made their appearance; and their relations to one another were unexplained. Indeed, the field was new, and gave promise of valuable results in behalf of the economic interests of the State.

To facilitate our review of the ground, I will at once introduce a vertical section of the strata with which we have to deal, presenting only such details as shall be essential to our purpose.

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It will be observed from the section that we have all three of the coal measure series represented in this region, but of the lower and upper measures only their upper and lowermost members are accessible. The middle coal- measures, however, here attains its full development, and it is with this division our attention shall be mainly occupied.

MIDDLE COAL-MEASURES.

The middle series of coal- measures, as developed in this part of the State, may be sub- divided into three groups, each of which possesses its own peculiar lithological characters in distinction from the other groups. The lower group has a thickness of from forty to seventy feet, the middle one eighty to one hundred feet, and the and the upper groups, probably, upward of fifty feet; making the average thickness of the middle series about two hundred feet. Towards the top of each of the groups the deposition of carbonaceous matter took place, forming as many distinct coal - horizons. The upper bed, coal C, is the most valuable one of the middle series. Its thickness varies between fifteen and thirty inches, and is a workable bed. It is overlaid by two to four feet of dark shales, abundantly charged with fossil remains which serve to distinguish this coal horizon from any of its associates. At the base of the upper group a well - marked carbonaceous bed is found, but it seldom presents any coal, though the highly carbonaceous black shale, capped by a single layer of compact argillaceous rock, is always met with wherever it has not been denuded. A bed of sandstone five to ten feet thick, forms the upper bed of the middle group, immediately beneath which the second carbonaceous horizon, coal B, occurs. This bed is very variable in thickness, ranging from one inch to two feet, and presenting even in short distances all the gradations from a black shale to coal of an excellent quality. It constitutes, however, a constant horizon, notwithstanding at certain localities it becomes so attenuated as scarcely to retain its identity. In the middle and lower portions of this group local developments of dark shales, sometimes more or less carbonaceous, are not unfrequent, beside there are bands of impure fossiliferous limestone, and the lower half is made up of clays and arenaceous beds. The lower

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