22 Barrett — Movements of the Strand Line. 



Fourth are the planation factors, working in opposition to 

 isostasy. They are the forces which tend to erode the lands, 

 fill the seas with sediment, and raise their surface level. Added 

 to this is whatever slow and secidar gain the ocean makes 

 in the volume of its water. 



The combinations of isostatic and compressive forces have 

 together operated through geologic time. The causes are com- 

 plex and the result is seen in the composite diastrophic rhythms 

 which are expressed in all ages hut which can be studied best 

 and more definitely in the record of the latest times. 



To sift apart the factors, comprehensive investigations must 

 be prosecuted over different parts of the earth. The sequence 

 and amount of oscillations in the tropics must be linked to 

 those of higher latitudes. Changes of sea level must be sepa- 

 rated from local changes in the level of the crust. Multiple 

 working hypotheses must be tested by the application of new 

 and significant facts. 



When, as Gilbert says in the introduction to his monograph 

 on Lake Bonneville, "the work of the geologist is finished and 

 his final comprehensive report written, the longest and most 

 important chapter will be upon the latest and shortest of the 

 geologic periods". 



