Page:The Journal of geology (IA journalofgeology21894univers).pdf/198

the land side and a somewhat less sinuous line on the seaward side; on the one hand were probably low hills and uplands sending out spurs here and there which cut off one marsh from another and often allowing long open stretches of low upland to reach out even to the waters of the sea itself; on the other hand were often narrow coastal plains rising scarcely above sea level, but, to a great extent, shutting off very effectually the saline waters from the swamps. Viewed areally, the productive portion of one of the great coal horizons is a wide irregular zone running in a tortuous course around a more or less extensive portion of the margin of a coal bearing basin included within the limits of a geological province. Examined at the present time coal horizons present, with all the irregularities of original deposition, subsequent change and deformation, a quite different aspect from the ideally perfect level of the ancient surface or zone which existed during the period of formation. In one direction, parallel to the shore, there is a series of minor saucer-shaped basins strung along on about the same great stratigraphical plane. They may rise or fall as the other strata change in inclination. They may be separated by wide stretches of sandstone or shale, or may come together in places. In the different basins the original vegetable materials in becoming compact shrink most in bulk in the middle, thus allowing the margins to remain considerably higher than the center. This is more noticeable in small basins than in large ones. Then, too, the fact that the direction of minimum movement in the changes of level was, as has been shown, parallel to the prevailing trend of the shore does not preclude even in this direction a certain amount of tilting of the strata either by the rising or the sinking of one portion of the shore more rapidly than another; or by the passage of some of the minor folds in directions not strictly harmonious with the general movement.

When a new cycle of vegetable accumulation took place the coastal swamps would again spread out at sea level, but not necessarily on planes exactly parallel to the horizon previously formed. Horizons which were separated to very considerable distances by