REGRESSIVE MOVEMENTS 733 



Local temporary increase in the rate of supply may be clue to 

 causes not readily determinable, but widespread and persistent 

 changes must be regarded as indicative of climatic change. Be- 

 fore we accept such a climatic change, however, as the cause of a 

 seaward migration of terrigenous deposits we must satisfy our- 

 selves, if possible, that the migration in question is not due to a 

 change in the rate of subsidence, the climatic conditions and, there- 

 fore, the rate of supply of detritus remaining the same. For it is 

 evident that a gradual diminution of the rate of subsidence would 

 produce practically the same results as a corresponding increase 

 in supply. 



II. Stationary Sea-Level. 



When the supply of detritus continues uniformly, while the 

 sea-level remains stationary, a regression of the seashore will take 

 place, but at a faster rate than would be the case if subsidence 

 still continued, though at a diminished rate. The shore zone would 

 creep out over the deeper water deposits, the transition from the 

 one to the other being rather more abrupt than in the case of a 

 slowly subsiding sea-floor. On the whole, however, stationary 

 conditions produce a change of sediment differing in degree only 

 from that incident upon a diminution in the rate of subsidence. 



III. Falling Sea-Level, 



The falling sea-level or rising land block is accompanied by a 

 continuous regression of the seashore, and a consequent seaward 

 migration of the shore zone with its attendant deposits. If the 

 emergence is not too fast the waves will be able to remove much 

 of the formerly deposited shore detritus and carry it seaward into 

 the shoaling off-shore districts. Thus a bed of sand or conglom- 

 erate will advance seaward over the finer off-shore deposits, com- 

 ing to rest upon these often without transition beds. Further- 

 more, the continuous movement of the clastic shore derivatives 

 will tend, in the coarser material, to a perfect rounding off of the 

 pebbles, and in general to a destruction of all but the most resistant 

 materials. Thus a much washed-over sandstone or conglomerate 

 may come to consist entirely of quartz, constituting a pure silica- 

 renyte or silicirudyte. It is probably not saying too much that all 

 pure quartz elastics, derived from a complex crystalline old land, 

 and resting abruptly upon a clayey or calcareous off-shore deposit, 



