DU ORM Ae 525 
is the lifting of the sea surface, whether by filling, by the spread- 
ing of the continent in its slow movement towards isostatic 
equilibrium, or by changes in the sea bottom. The effects in 
any case are essentially the same, and are world-wide by reason 
of the common level maintained by the ocean in all its parts. 
This sea lifting combines with and modifies both the terrace 
building and the shore cutting, and the common result is a shelf 
occupied by a shallow sea. This shelf is the great theater of 
sedimentation and of littoral life evolution. Its peculiar con- 
figuration, by giving great breadth to the shallow water circum- 
continental seas upon a slight lifting of the sea level, or after 
the erosion and terrace building of prolonged quiescence, on the 
one hand, and by narrowing these shallow seas to mere fringing 
ribbons upon the drawing away of the sea until its shore stands 
against the abysmal edge of the shelf, on the other, makes it a 
vital factor in geological progress and gives occasion for a spe- 
cific designation. 
It is, however, desirable to exclude those areas that become 
submerged by their own individual movements and take on the 
similitude of submarine terraces without having any genetic or 
systematic relation to the sea level as such. They may stand at 
such depth as to give an expansion of shallow water just when 
the withdrawal of the sea narrows the shallow water tract on the 
true genetic shelf, and thus they may antagonize the evolutionary 
effects of the latter upon littoral life. They may, to be sure, 
coincide so nearly with the true shelf in position as to work con- 
currently with it and increase its effects, but this, from the nature 
of the case, will rather be the exception than the rule. Such 
adventitiously submerged portions of the continent must be 
regarded as factors that vitiate the ideal workings of the true 
sea-generated terrace. 
Both the true sea-formed terrace and the continental border 
submerged by subsidence are at present embraced without 
distinction under the phrase ‘continental shelf ’’—a designation 
that fairly represents the topographic fact, but does not carry 
with it any specific idea of the diverse agencies involved in its 
