Page:The Journal of geology (IA journalofgeology11893univers).pdf/54
Or, on the other hand, moderate uplift of the base-leveled continent, must cause the revived streams rapidly to sweep into the sea the mass of insoluble clay and sand which formed the residual mantle. Thus the limestone deposits will be succeeded by a thickness of shales of a more or less arenaceous or clayey character.
From these considerations it follows that a complete topographic cycle may be related to a sedimentary sequence composed of a sandy base, a limestone middle and a shale top. Newberry first noted the frequent recurrence of this sequence, and sought an explanation in conditions related simply to the sea; its advance, presence and retreat. When he made his generalization the base-level had not been recognized as a result of continued erosion, nor had Gilbert analyzed the process of erosion; and Davis had not described a topographic cycle. These contributions to the science have widened the field of inference, and the topographic phase of the land can no longer be disregarded in the discussion of the deposits of the sea.
But it should not be forgotten that the inference from sediments should be confined to the topographic phase of a belt of land extending back from the shore to a moderate distance only. The products of rock-breaking disintegrate during prolonged transportation and mountains remote from the coast are not indicated in deltas of great rivers. A student of the deposits of the Mississippi would not infer the height of the Rocky mountains, but the sands of the Klamath river bear witness to the nearness of the coast range.
The analysis and discussion of conditions which govern the character of the material contributed from land to sea might be extended in detail, and illustrated by descriptions of sediments in existing rivers, but the subject is worthy of independent treatment.
SEDIMENTATION.
Sedimentation consists of three sub-processes, sorting, distribution and deposition. These are effected by waves and undertow, tides, winds and oceanic currents and are modified