DIASTROPHISM AND THE FORMATIVE PROCESSES 197 
ate the primary movement and give it a cumulative value. This 
tends to push the movement to a maximum the better to prepare 
the way for a new quiescent period. 
Such cumulative periodicity, both primary and enhanced, seems 
not only consistent with a solid elastic earth but an inevitable 
consequence of the elastic rigidity of such an earth. As already 
remarked, the normal mode of isostatic adjustment in such an earth 
is thought to be wedging action in the form of movements on the 
part of its constituent tapering prisms, conical, pyramidal, or 
otherwise, in response to the varying stresses imposed on them. 
Facilities for such movements are presumed to be provided by 
vertical schistosity developed in the tracts where the differential 
stresses are greatest, and by the very stresses that actuate the deep 
diastrophic movements. So originated they should reach to what- 
ever depths may be seriously affected by differential stresses of 
an order requiring readjustment. No undertow in a hypothetical 
mobile substratum is necessarily involved and none is postulated. 
The weighted parts wedge down and the unloaded parts are wedged 
up until the differential gravitative stresses are essentially equated 
and an isostatic state reached. Movement in a rigid earth of 
course is not presumed to take place until stresses have accumulated 
toa degree adequate to force it and hence the relatively quiescent 
stages. The quiescent stages occupied in such stress-accumulation 
are interpreted as constituting the periods marked by base-leveling 
and sea-transgression, appropriate conditions for which are thus 
provided. General isostatic readjustments are of course interpreted 
as synonymous with general diastrophic movements and so are 
regarded as equally periodic. As the present epoch has been pre- 
ceded by great diastrophic movements, the earth body is presum- 
ably now in a stage of approximate isostatic adjustment, as 
implied by geodetic evidences. 
