Page:The Journal of geology (IA journalofgeology21894univers).pdf/192

The Coal Measures of the Mississippi valley occupy an area of more than 120,000 square miles. They extend in an almost unbroken field from the western flank of the Appalachians on the east to beyond the Missouri river on the west. Separating the area into two somewhat unequal parts is the Mississippi river along the course of which are exposed older rocks. The Coal Measures may have been at one time continuous over the central portion, but the outcrops of the more ancient rocks along the borders of the Mississippi is probably not due entirely to unaided erosion but in part to slight folding, an anticlinal axis coinciding approximately with the line of the great river.

The geological history of the eastern and western areas, which are sometimes called respectively the Central and the Western Interior coal fields, is probably similar, though in some particular phases there is a divergence which dates back prior to the close of the Lower Carboniferous. The northern portion of the Coal Measures west of the Mississippi forms a broad bay-like expansion opening to the westward. Beyond the Missouri river the strata are hidden from view by newer sediments. The most productive portion of the Western Interior coal field is a marginal zone extending from northcentral Iowa southeastward to northeastern Missouri, thence sweeping to the westward around the Ozark uplift into Indian Territory and continuing on into central Texas. The interior portion of the bay-like expansion of Coal Measures is as a general thing unproductive, though a few thin seams of coal do occur.

Everywhere throughout the region the stratigraphical details present great simplicity, being almost free from the effects of orographic movements. The lithological characters of any one locality are repeated again and again in the same monotonous