DIASTROPHISM AND THE FORMATIVE PROCESSES 525 
3. The sea-shelf 1s definitely correlated with the penetration of solar 
rays. This is a relation of biologic moment. As the mode of 
formation keeps the face of the shelf within a certain distance from 
the sea-surface, the waters upon it have a rather definite range of 
illumination. Competent opinion places the larger part of the 
effective photo-synthetic action within a hundred meters of the 
surface, while photographic effects vanish at three or four hundred 
meters. Organisms that depend on insolation, or that live upon 
those that do, may be assumed to be rarely fossilized at greater 
depths than these when there is no ground to suspect postmortem 
transportation. The shelf-sea deposits therefore embrace the sessile 
photo-synthetic types and their dependencies. Deeper deposits 
may embrace the pelagic types, but rarely the sessile and quasi- 
sessile forms. The shelf zone is therefore from the very nature of 
the case a biologic horizon of the first importance. Its faunas in 
consequence belong to a distinctive type. The life entrapped in the 
bed of the shelf-sea is demarked from that caught in the abysmal 
bed of the ocean. 
There is, however, a narrow belt about the borders of dias- 
trophic seas that enjoys photo-vital conditions much the same as 
the gradational shelves, though the slopes are normally steeper and 
the life-conditions somewhat more special and precarious in general- 
A comparison of the relative values of the gradational shelves and 
the diastrophic belts within the same depth-limits is a critical part 
of this study and will be taken up presently. It thus appears that 
the shelf-seas are photobathic zones of special effectiveness, and are, 
and no doubt always have been, the habitat of a most important 
type of marine faunas and floras, the class most akin to the life of 
the land—pre-eminently the class on which the divisions of geologic 
history have been based, and may best be based still more specifi- 
cally, so far as these divisions are biologic. 
4. It is scarcely more than a reiteration of the last statement in 
a special form to say that the faunas of the shelf-seas of ancient 
times were given distinctive aspects by the conditions of insolation, 
of aeration, of low pressure, and of agitated bottom, all of which 
were determined by the mode of origin of these seas. These sea- 
conditions became, therefore, critical factors in the history’ of 
