98 PRESIDENT’S ADDRESS—-SECTION C. 
seems almost certain that it must In some way be due to the 
attractions of the sun and moon, for no other reasonable hypo- 
thesis can be suggested. No doubt the theory that oscillations of 
the surface are due to internal movements set up by bodily tides 
rests at present on no observational basis. But there are reasons 
for thinking that such internal movements are possible, or even 
probable ; and, if such movements do actually take place, the 
theory gives a simple and fairly complete explanation of the 
surface movements. 
In both regional and mountain upheaval we find a very slow 
subsidence followed by a comparatively rapid elevation, the 
principal difference between them being that in a mountain 
range granite has broken through the crust and the sedimentary 
rocks have been metamorphosed and plicated, while in regional 
upheaval there is no visible granite and the sedimentary rocks 
have been stretched. These two types, it must be remembered, 
are not isolated but connected by others, which thus afford 
additional evidence of the unity of the elevatory force. If the 
lateral pressure, the effects of which are plainly visible in 
mountain ranges, is caused by expansion of the sedimentary 
rocks, as the gradation theory supposes, then it must be quite 
a different thing from the vertical pressure which causes regional 
uplifts, and there is no reason why the two should so often 
occur together. But if the lateral pressure be due to the irrup- 
tion of granite, then the force which causes uplifts causes lateral 
pressure as well. In other words, the only difference between 
the two kinds of uplift is that in one the molten interior has 
broken through the crust and contorted it, while in the other 
case it has not done so. 
The cause of the difference between the two kinds of uplift 
seems to be the amount of previous sedimentation on the area. 
If sedimentation has been very heavy the crust is, m some way, 
so weakened that the granite breaks through. But if previous 
sedimentation is not heavy the crust remains sufliciently strong 
to resist breakage, and a series of oscillations may follow, until 
at last the sediments attain the necessary thickness, the crust 
is broken through and permanent elevation takes place. This 
explains why regionai uplifts followed by subsidence have often 
preceded the mountain uplift which terminates for a time the 
series of oscillations. If it should be proved that areas of 
elevation and depression always lie alongside one another, and if 
this holds for ocean beds as well as for continents, it will go far 
to prove the theory, for there is no other explanation but that 
of internal movement. 
I have thus tried to give you as complete an account as I can 
of the present position of theory on this fundamental problem of 
dynamical geology. You will notice that during the last fifty 
years investigation has been more destructive than constructive, 
