SOURCE OF EVOLUTION OF PROVINCIAL FAUNAS 601 
favorable to an expansional evolution of this shallow-water 
marine life are those which ensued upon a protracted period of 
base leveling.* This, by its very terms, implies a protracted 
period of freedom from great movements on the part of the 
land or the sea. At the climax of such a period there is nor- 
mally an extensive transgression of the sea upon the continental 
platform which assumes two phases: (1) the development of 
broad sea-shelves by the cutting back landward of the sea cliff 
and the building out seaward of the submarine sea terrace by 
means of the land detritus ; (2) the creeping of the sea waters 
far inland upon the lower portions of the continent. 
At first thought it may be questioned whether the land will 
not be extended by the addition of detritus to its border, and, 
still further, whether the transgression of the sea is genetically 
connected with base leveling and is its normal attendant. That 
the land is now being extended in certain places by detrital 
accretions to its borders, as in the case of deltas, is beyond 
question, but it is equally beyond question that the sea is 
advancing in other places, and it will probably be apparent, after 
a careful inspection of the continental coasts, that on the aver- 
age the sea is advancing rather than retiring. But the present 
is far removed from a base-level period. The streams carry to 
the sea much more detritus than they would were the surface 
closely approaching a base level. The sea also, it is to be 
admitted, is better able to carry detritus back to deep water 
under present conditions than it would be if its sea-shelf were 
greatly extended. Conclusions drawn from present conditions 
are, therefore, embarrassed to this extent on both sides. The 
issue is really a contest between the ability of the streams to 
deliver detritus at the coast line and the ability of the sea to 
carry it back to deep water. The delivery of the streams is a 
declining factor which approaches zero as the base level is 
approached. The carrying ability of the sea is much more 
tA suitable sinking of the land independent of base leveling may produce similar 
though not quite identical results, but in so far as this is adventitious it does not fall 
into the category under discussion here. 
