616 NNO (CLELAND OEPEIEICSUN 
carbonic acid were reduced temporarily by the indirect adverse 
influences springing from the impoverishment of the life on the 
land, the continued reduction of the rate of consumption of car- 
bonic acid on the land must cause it at length to fall below the 
freeing action in the sea, and the impoverishment of the atmos- 
phere give place to enrichment which would run its course until 
the preponderance of action was again reversed. 
But another element of vital importance enters the problem, 
the changing attitude of the land and the sea. Such changes 
may be systematic or adventitious. If they are adventitious the 
results are beyond easy discussion, but adventitious changes are 
believed to be subordinate to the systematic changes, since these 
latter follow (1) from codrdinate movements of the earth’s 
crust; (2) from uncoérdinate movements of the crust which the 
ocean codrdinates by its leveling function, and (3) from no 
movements at all. The last is the simplest and most represent- 
ative case. 
Let there be no essential movement of the crust for a pro- 
longed period. That this has been an actual case repeatedly, the 
base levels of different periods testify. During such a period 
the height and the area of the land are both diminished. The 
rate of disintegration of the rocks is consequently reduced, and, 
concurrently, the rate of impoverishment of the atmosphere in 
respect to carbonic acid and of the conveyance of calcium bicar- 
bonate to the ocean’ is also diminished. At the same time the 
edge of the sea is advancing upon the borders of the land, partly 
by erosion, partly by the lifting of the sea level by the recep- 
tion of sediments, and partly, perhaps, by the quasi-fluent creep 
of the continent toward isostatic equilibrium.? This results in 
the extension of the sea shelf and possibly in the formation of 
interior epicontinental seas, as exemplified notably in the central 
epochs of the Ordovician, Silurian, Devonian, Carboniferous, 
and Cretaceous periods. This extended sea shelf and these epi- 
«The Ulterior Basis for the Classification of Geologic Time Divisions. Jour. 
GEOL., Vol. VI, No. 5, 1898. 4 
218ocnClt 
