566 TNC ACGHAMBERLITN 
elevation appear to be well authenticated by present geological 
data, imperfect as they are for certain quarters of the earth. 
These periods occurred near the close of the Paleozoic and of the 
Cenozoic eras respectively. Closely connected with these two 
periods are two well authenticated glacial periods. 
If, on the other hand, there were periods of prolonged 
quiescence of the earth’s body during which the lands were cut 
down well towards base level, they would be accompanied by a 
progressive slackening of the rate of disintegration, and a corre- 
sponding reduction in the rate of atmospheric loss of carbonic 
acid, giving opportunity for the agencies of repletion to over- 
take and surpass the agencies of depletion. It is believed that 
there was a series of such periods among which the Cretaceous 
is best authenticated. 
During a prolonged period of relative quiescence the 
encroachment of the coast lines upon the land, if elevated, 
becomes notable. In addition to this, the material removed 
from the land and deposited in the sea raises the water level, 
and when the degradation is notable this rise amounts to an 
appreciable factor. This aids the coast action by lifting it 
above the restraining influences of its own products, which, by 
shoaling the water off shore, tend to break the force of wave 
action. The notion is also entertained that during periods of 
readjustment the continental masses are apt to be lifted beyond 
the plane of perfect isostatic equilibrium, and that there follows 
a tendency to slowly creep back into equilibrium, accompanied 
by a general tendency of the continental platform to flatten out 
under gravitative stress. The continental platforms are to be 
regarded as having elevations above the abysmal ocean bottom 
of perhaps 12,000 feet, and, as their average gravity is two and 
a half to three times that of the water which surrounds them, 
there remains a large excess of gravitative stress tending to 
cause the continents to spread laterally. 
The combined effects of these agencies is a transgression of 
a thin edge of the sea upon the land. Now, such extensive 
transgression took place on all the great continents in the Upper 
