598 T. C. CHAMBERLIN 
oidal surface to which an ideal earth would be adjusted lies 
about g000 feet below the ocean surface.'. The portions of the 
ocean basin below this normal level represent an excess of 
shrinkage. The continental masses which stand above this 
average level represent a deficiency in shrinkage. This average 
level is the natural datum plane from which the continents may 
be conceived to rise. The ideal upper surface of a continent 
may be said to be the sea level, a plane which the upper surface 
of the continent constantly approaches but never entirely 
reaches. That portion of it which is exposed above the sea 
level undergoes constant truncation by air and water. The por- 
tion beneath the sea level is being constantly built up by the 
deposition of land wash about its borders, forming a sea shelf 
whose summit plane is the sea level.? As a result of these activ- 
ities, continued throughout the ages, the continents have come 
to be approximate platforms whose theoretical upper horizon is 
the sea level. To this they are accommodated more or less 
approximately, but never perfectly. They reach their most 
complete adjustment after long intervals of relative quiescence, ~ 
when base-leveling attains its highest degree of perfection. 
They depart most widely from the theoretical surface at the 
climax of great periods of crustal readjustment to accumulated 
internal stresses. Such periods of greatest departure inaugurate 
periods of maximum activity, both on the part of the leveling 
forces, whose function it is to reduce the land surface again to 
the sea level, and on the part of the depositional agencies whose 
function it is to build up a sea-shelf around the borders of the 
continent. In other words, agencies for the replanation of the 
platform are put into maximum activity by the very agency 
Which deformed it. If we conceive the continental platform to 
be the basal portion of a broad truncated pyramid whose bottom 
rests upon the ideal average level, go00 feet below the sea sur- 
face, and whose truncated summit is ideally at the sea level, it 
‘GILBERT, Article ‘Earth,’ Johnson’s Cyclopedia. 
2See “The Ulterior Basis of Time Divisions and the Classification of Geologic 
History,” Jour. GEOL., Vol. VI, No. 5, 1898, pp. 449-462. 
