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in Ohio and Pennsylvania, as the mountains are approached, the structure increases rapidly in complexity, until in the highly folded and faulted districts attempts to follow out the original state of things may become utterly hopeless.

The majority of the larger coal deposits of the Western Interior field may be considered then as having been formed in swamps skirting a great shallow gulf, the extent of the produc-

Fig. 4.Coal Horizon viewed at right angles to shore line.

tive portions of the different horizons being in a measure dependent upon the length of time the physical conditions were favorable to coal formation. Many short minor episodes doubtless existed between the larger ones, during which comparatively small accumulations of vegetable material took place.

Another fact to be taken into consideration is that all the coal of the region was not formed in marine swamps, but that some of the minor basins were doubtless originally a very considerable distance from the sea, while certain others were formed where open sea conditions prevailed largely. A few seams also appear to have been formed as drift materials in estuaries at the mouths of streams.