THE ULTERIOR BASIS OF TIME DIVISIONS 455 
ances from local and from general sources. There appear to be 
two systematic sources of slight but very critical modification 
that require special consideration. 
Fic. 2.—é # Continental base-plain. s7Sealevel. ¢/ Submerged terrace plain. 
a ¢ Abysmal terrace face. 
1. The transfer of débris from the land to the sea displaces 
an equivalent amount of water, and raises the sea level propor- 
tionately, and causes an advance upon the land. The effects 
are volumetrically small compared with the great body of the 
ocean, but a slight rise in the surface as the baselevel stages of 
the continent are attained is peculiarly effective. This codperates 
with the cutting back of the sea cliff, and, combined, they 
become effective in advancing the edge of the sea upon tlie 
border of the land. 
2. There are both theoretical and observational grounds for 
the belief that in the process of periodic readjustment of the 
earth to its internal stresses, portions of the crust are thrust up 
to heights notably above the plane of isostatic equilibrium, 
and that these portions gradually settle back toward equilibrium 
by virtue of the slow fluency or quasi-fluency of the rocks. 
Recent pendulum studies by Putnam and Gilbert seem to indi- 
cate that the portion of our continent most notably lifted in late 
Tertiary times still stands appreciably above isostatic equilibrium, 
and there is little doubt that the same is true of other continents, 
as is, indeed, indicated by partial pendulum data. There is, 
however, a large mass of concurrent data which shows an aggre- 
gate subsidence of the continent since late Tertiary times, data 
which have been industriously marshaled in the interests of an 
epeirogenic explanation of the glacial period. This leads to the 
impression that in late Tertiary times, when the upward move- 
