DIASTROPHISM AND THE FORMATIVE PROCESSES 679 
There are of course other methods of determining the nature of 
the earth-body, such as the modes of the seismic vibrations that 
traverse the earth, the time-relations of the body-tides to the passing 
of the tide-producing bodies, and so forth. The testimony of these 
is indispensable to a full consideration of the nature of the earth- 
body, but we are here considering merely the bearing of the strictly 
geological arguments that spring from gradational phenomena. 
As previously urged, a base-level of wide prevalence is practically 
impossible if diastrophic movements are in continuous progress, for 
warping and dislocation are inherently inimical to mature plana- 
tion. So too in shelf-building, the range within which wave action 
is effective in distributing sediments, and the depth within which 
light is effective in photo-synthesis, are so narrow compared with 
the ranges through which warping and dislocation shift the crust 
that relatively little diastrophism would forestall the formation of 
great parallel sets of strata and wide continuity of shallow-water life. 
This will become clearer, and certain additional considerations 
will be brought into view, if we follow out systematically a normal 
evolution under each of the two types now basally distinguished. 
The systematic evolution of a continent under continuous 
diastrophism is most easily followed if we accept the view that the 
mean specific gravity of the continental masses is lower than that of 
the suboceanic masses and that a certain tendency to isostatic 
equilibrium between the two arises from this difference of specific 
gravity—a view strongly supported by concurrent lines of evidence. 
Probably all will agree to this in some degree at least, whatever 
limitation or qualification they may wish to impose upon it. If this 
be accepted, it is not material te our immediate purpose whether 
the diastrophism is actuated by internal changes of volume, by the 
transfer of matter and energy from below to the surface, by the 
transfer of matter from higher to lower levels on the surface, or by 
all combined, or by other agencies, for the trend toward isostatic 
equilibrium permeates all and, to the extent of its value, gives shape 
to their effects in accordance with mechanical laws. The portions 
of the crust that are already bowed outward, and are at the same 
time lighter than the average, are inevitably bowed upward under 
further stress, and the portions that are already sagged, and are at 
