Page:A Treatise on Geology, volume 1.djvu/207

 CHAP. VI. diffusion in the ocean from an area situated to the south-east; the shale transported from the west, ami the sandstone, plants, &c., drifted from the north. We may imagine two rivers, one flowing from the west, and bringing across the regions where now are Ireland, Lancashire, Derbyshire, and South Yorkshire, a vast body of argillaceous sediments, slightly charged with sand, and but little varied by floating trees and plants; the other rushing from the north, loaded with sandy matter, and bearing abundance of trees of different kinds, but not many ferns or delicate herbaceous plants. Alternately or contemporaneously, these rivers might fill the sea with deposits, such as we behold and in the manner that we see them, united with the proper calcareous deposit of the ocean.

This explanation of different sediments coming to the same part of the sea from various quarters, may probably be applied to every system of stratified rocks, containing, as constituent members, limestone, sandstone, and clay; but it is necessary previously to investigate the directions in which the agencies concerned in producing each sort of sediment were most powerful; i.e. the points or lines of their greatest intensity.

In some cases it appears highly probable that one such irregular fluviatile action, modifying the continuous depositions from the sea, would sufficiently explain the phenomena of the association of sandstone, shale, and limestone; because, by such action, the shores would be margined by a sandy deposit, beyond which clay would predominate in the sediments, and at a greater distance calcareous matter would be nearly unmixed with the effects of littoral agitation.

In the diagram No. 53. S represents the sandy accumulation near the shore, passing by gradation to the