176 PROCEEDINGS OF THE AMERICAN ACADEMY. 



on, and at a late stage will show as little elevation as was seen in the 

 youthful nip. 



While the sea has produced the submarine platform, the land has been 

 worn down by subaerial degradation to a peneplain.* The controlling 

 plain for the production of the peneplain surface is baselevel, " the level 

 of the sea . . . below which the dry lands cannot be eroded." f The 

 surface will never reach baselevel, but will approach it, " in an infinite 

 series of approximations like the approach of an hyperbola to tangency 

 with its asymptote." t A possible qualification of the above statement 

 may be, that where the surface is near baselevel, the wind may excavate 

 a portion down to or even below sealevel. 



American and English Views. % — Major Powell, Major Button, Mr. 

 Gilbert, and other geologists who worked upon our western interior 

 region, saw the great importance of sea-level as the controlling baselevel 

 down toward which the laud is worn. The action of the sea did not 

 enter into their considerations to any extent. The English geologists on 

 the otlier hand saw upon tlieir island the great destruction wrought by 

 the waves, and the lower level of wave action was their most important 

 plane of reference. Professor Ramsay included the subaerial forces as 

 aids in marine denudation, while later Dr. Geikie || made sea cutting of 

 less importance than subaerial denudation in the production of the plain 

 of marine denudation. 



Figure 5. S L = sea level. \VB = wave-base. P = peneplain. 

 S P = submarine platform. CD— continental delta. 



Wave- Base. — The twb planes of control should be distinguished, and 

 the almost plains produced by subaerial and submarine degradntion be 

 given separate names. Figure 5 shows the relation of the peneplain 

 surface with its controlling baselevel to the submarine platform and its 



* W. M. Davis, A. J. of S., 1889, XXXVII. 430. 



t J. W. Powell, Exploration Colorado River of the West, 1875, 203. 



t C. E. Dutton, Tertiary History of the Grand Canon District, Mon. II., 

 U. S. G. S., 1882, 76. 



§ Since the following section was written, Professor Davis has made a more 

 extensive comparison of the American and English schools; Bull. G. S. A., 1896, 

 VII. 377-398. 



II Scenery of Scotland, 1887, 137. 



