Submergences and Emergences of the Continents. 485 
companied by the emergence of all other land areas which were 
not under a greater depth of water, unless they too^subsided 
more rapidly than the land area which emerged. After such 
land areas emerged they would be eroded. After this, as a re¬ 
sult of their erosion or subsidence, one or more of the land areas 
might be submerged. The submergence of one area would not 
necessarily result in the submergence of all which emerged at 
the same time, for the subsidence of land areas may be differ¬ 
ential. However, the land areas which emerge at the same 
time might finally be again submerged. If this idea of simul¬ 
taneous emergence of land areas in different continents and their 
subsequent submergence, either simultaneously or successively, 
be true, it would follow that certain unconformities are inter¬ 
continental. It is to be noted that such partial continental 
equivalence of unconformity is producible by the initial subsi¬ 
dence of the sea bottom. The greater breaks in the geological 
succession give a certain amount of confirmation to this idea of 
intercontinental unconformity. Some of the great interconti¬ 
nental unconformities are (1) the break at the base of the Cam¬ 
brian, (2) the break between the Upper Silurian and Lower Si¬ 
lurian, or between the Silurian and Ordovician, and (3) the 
break between the Mesozoic and Paleozoic. 
It is not supposed that the above general statement is com¬ 
plete. There are various disturbing factors which make the 
problem of differential subsidence, resulting in positive or neg¬ 
ative movement of the sea shore, very complicated. One of the 
greatest of these disturbing elements is the lengthening day. 
It has already been seen that the rate of rotation of the earth 
has been decreasing for millions of years. Concurrent with 
this, there has been a decreasing oblateness. Blytt 1 explains 
that the water of the ocean would adjust itself at once to the 
changing speed, and as a result of the lessened centrifugal force 
would fall at the equator and rise in the polar regions. He 
further argues that the rigid earth would lag behind in its ad¬ 
justment, until the stresses accumulated so as to overcome the 
rigidity of the rocks, when movement, once begun, would take 
1 A probable cause of the displacement of beach lines, by A. Blytt: 
Christiania, 1889, p. 89. 
