170 PROCEEDINGS OP THE AMERICAN ACADEMY. 



Variations. — As in the case of uniform uplift (p. 165) there will be 

 great variations from this ideal scheme of criteria for uniform depression. 

 The stage of development interrupted by the drowning, the steepness 

 and structure of the coast, and the rate of submergence, all have impor- 

 tant bearing upon the form of the depressed coastal and shore forms. 

 Slow sinking while the sea cuts into the land will materially aid the for- 

 mation of a planation surface. Professor von Richthofen goes so far as 

 to consider all regional plains of abrasion, " Abrasiontlachen/' as neces- 

 sarily the work of the sea aided by slow submergence.* 



The gradual depression and cloaking over of a region are the normal 

 results of the isostatic return to a condition of equilibrium. Stripping 

 in one area and loading in another causes a lack of balance, which will 

 be restored by a rising of the stripped, and a sinking of the loaded area. 

 One of the best examples of isostasy is seen in the Mississippi basin. f 



Now while the principle of isostasy explains some regions of slow 

 depression with concomitant sedimentation, it does not account for the 

 more pronounced changes of level, introduced by secular elevation or 

 depression. Geographic cycles are not introduced by isostatic move- 

 ments. The suggestions of cause are numerous, but these geological 

 questions are not considered in this paper. The subject is here dismissed 

 with the statement, made by Major Button, that " the nature of the process 

 is, at present, a complete mystery." % 



4. Diverse Movements. 



Tilting ; Position of Pirotal Axis. — Uniform uplift and depression 

 have been considered, and the resulting initial forms contrasted in the 

 two cases. If, instead of a uniform uplift throughout the area, the move- 

 ment is diverse, we have tilting, warping, or crumpling and faulting. If 

 the change of quantity proceeds at a constant rate, we have rigid tilting; 

 if at a variable rate, but of moderate variety, we have warping ; while if 

 much irregularity of rate appears, we have disorderly crumpling or 

 faulting. 



With the exception that the topographic forms are elevated or de- 

 prissed to different amounts in various places, the criteria of tilting are 

 the same as those already discussed. Tilting may be of such a character 



* Fiibrer fiir Forschungsreisende, 1886, 354. 



t See the following artides : McGee, A. J. of S., 1802, XLIV. 177-192; Bull. 

 G. S. .\., 1804, VI. 5.5-70 ; Keves. Bull. G. S. A., 1894, V. 231-242. 

 t Pliil. Soc. Wash., 1889, XI. 03, G4. 



