THE SILURIAN FAUNA INTERPRETED ON THE 
EPICONTINENTAL BASIS. 
THE oceanic movements which brought the Ordovician period 
to a close are believed to have been such as to affect all conti- 
nents ina similar manner. The transition period from Ordovician 
to Silurian was probably characterized by a special shrinkage of 
the earth, due to an effort at adjustment to the stresses that had 
been accumulating during the whole of Cambrian and Ordovician 
time. In this shrinking process it is assumed that the ocean 
basins were made deeper and their capacity increased so that 
the waters of the shallow seas lying upon the borders of the 
continents and reaching into their interiors to some notable 
extent, were drawn off, and the bottoms of these seas became a 
part of the dry land. It is assumed that the continental shore 
lines migrated oceanward until they no longer lay upon the 
continental platforms themselves, but upon their abysmal slopes, 
and the former broad, shallow-water tracts of the sea-shelves 
were reduced to narrow bands. With the destruction of these 
shallow seas upon the continental platforms, the multitudes of 
shallow-water organisms which had existed in them were largely 
forced into extinction.” . 
After the readjustment of the solid earth, the seas began 
again gradually to creep upon the continental platforms by 
means of the landward cutting of the sea-cliffs, by reason of the 
sediments carried down from the land and dumped into the 
ocean basins, and by reason of the gradual settling back of those 
portions of the crust which had been locally forced upward 
beyond isostatic equilibrium. With the continuation of these 
processes new sea-shelves and new epicontinental seas came into 
t For a fuller exposition of this hypothesis see ‘‘ The Ulterior Basis of Time Divi- 
sions and the Classification of Geologic History,” by T. C. CHAMBERLIN, Jour. 
GEOL., Vol. VI, p. 449. ~ 
692 
