480 THE JOURNAL OF GEOLOGY. 



Or, on the other hand, moderate uplift of the base-leveled conti- 

 nent, must cause the revived streams rapidly to sweep into the 

 sea the mass of insoluble clay and sand which formed the resi- 

 dual 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 gen- 

 eralization 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 in- 

 ference, 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 sedi- 

 ments 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 in- 

 dicated in deltas of great rivers. A student of the deposits of 

 the Mississippi would not infer the height of the Rocky moun- 

 tains, but the sands of the Klamath river bear witness to the near- 

 ness 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 treat- 

 ment. 



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 



