Lawson. I 



Po$t-Phocene Diastrophism. 



'59 



archipelagic condition endured into the early Pleistocene, and from 

 this condition it has been gradually recovering up to the present 

 day. The Channel Islands may be regarded as the remnants of this 

 late Pliocene and early Pleistocene condition. The uplift has robbed 

 the coast of its ancient harbors, and the only notable one of the 

 present day exists by grace of special and local, but very vigorous, 

 orogenic movements on the Peninsula of San Francisco. With this 

 radical transformation in the physiography of the coast, there have 

 doubtless been very important changes in the climate, and in this 

 fact is to be found, probably, the explanation of certain remarkable 

 and anomalous features in the distribution of the plants of the. coast. 

 A map of the shore at the beginning of the Pliocene would resem- 

 ble the one of to-day. A map at the beginning of the Pleistocene 

 would resemble rather that of the present Alaskan shore-line. 



In the continuous chain of geological events from the inaugura- 

 tion of the Pliocene to the present time, it is not an easy matter to 

 delimit the Pliocene and Pleistocene epochs so that they shall cor- 

 respond to the same divisions of the geological scale in the eastern 

 part of the continent. The difficulty is the greater if we depend 

 entirely upon palaeontological data; and, probably, a satisfactory 

 definition of the Pleistocene of the west coast as distinct from the 

 Pliocene, will never be formulated upon a purely palaeontological 

 basis. The reason for this is that there has been no distinct break 

 in the continuity of marine conditions throughout the epochs, only 

 a gradual transition of conditions. In this gradual transition there 

 was, however, a reversal of the epeirogenic movement of the coast 

 from a process of depression to a process of uplift. This turning 

 point of the diastrophic pendulum, the initiation of the diastole of 

 the epeirogenic pulsation, is believed to correspond well with the 

 beginning of the Pleistocene. The two epochs thus delimited have 

 no interval of erosion between them, and there will be found no 

 marked break, except locally, in the sequence of marine life. On 

 the other hand, the Pliocene is separated from the Miocene by an 

 important interval of erosion. It follows that the subdivision of 

 the Tertiary, or Cainozoic into Eocene and Neocene is not suited to 

 the west coast. The Miocene is distinct. The Pliocene merges 

 into the Pleistocene, and the marine strata of the two epochs must, 

 to a large extent, be mapped together. 



