84 
GEOPHYSICS: G. F. BECKER 
water falling on a continent and carrying sediment to the sea depotential- 
izes or sets free the energy of position acquired at the expense of the heat 
stored in the earth. 
At great depths we know that rocks are deformed and undergo soKd 
flow; and it is well known that under an appropriate system of stresses 
any solid must fiow.^ At the surface, so far as rocks are concerned, such 
a system of stresses does not exist, and the rocks do not flow. But erosive 
action lends them a mobility almost equivalent to fluidity so that the 
net result is in some respects analogous to that which would ensue if the 
solid surface of the globe were replaced by a mass of hyperviscous liquid 
some column or columns of which had a higher temperature than the 
surrounding matter. These columns would rise above the general sur- 
face because of the diminished density and the mounds thus formed 
would overflow or run down because they lack rigidity. The outflowing 
portions would cool, and sinking into general mass, would estabhsh a 
convective circulation. 
Not just in the same way, but similarly, erosion effects the flow of the 
continental surface matter to or beyond the edge of the continental 
plateaus overweighting the ocean floor and bringing about a correspond- 
ing subsidence. 
In an asphalt lake like that of Trinidad, convection due to lack of 
temperature equihbrium would be attended by an undertow. Material 
rising from any particular depth would diminish the horizontal pressure 
which it had previously exerted on surrounding portions of the hyper- 
viscous mass and these would press inwards to fill the partial void. In a 
solid earth there must be an analogous action, excepting that the partial 
pressure needful to produce lateral flow or undertow must exceed that 
which would strain the solid rock to its elastic limit. 
The analogy of an asphalt lake must not be applied without caution. 
In such a lake it is easy to conceive of convective circulation indefinitely 
continued. Not so in the solid earth. If the whole rock mass from which 
the oceanic salt has been derived was really once piled on the continents 
and if the ocean is 100 X 10^ years old then the total uplift of about 6| 
km. has only been effected at the rate of 1 mm. in 15 years or 1 inch in 
380 years. Thus the process might be compared with incipient con- 
vection in an asphalt lake. 
None the less, so far as it has gone, the undertow has tended to con- 
tract the area of incipient continents, to increase the crumpling and to 
exaggerate the elevation to which they would have attained had there 
been no convective tendency. 
