May 5, 1923] 
NATURE 
605 

solar gravitational attraction. Oceanic tides are com- 
paratively feeble phenomena ; for not only is the ocean 
shallow and obstructed by land, but also water is a 
fluid of low density. But during times of revolution, 
just beneath the continents and oceans, there comes 
into existence a vast and far deeper ocean, composed 
for a great part of a highly fluid substance having 
a density three times that of water. We seem to have, 
therefore, good and sufficient reason for expecting 
greatly intensified tidal phenomena to arise during these 
times. So also a precessional force must act with 
intensified effect in periods of revolution. Both these 
forces tend to retard the surface crust of the earth in 
_its diurnal rotation from west to east ; that is, they tend 
-to hold it back a little from partaking of the general 
easterly rotation of the globe. The effect is greatest 
in equatorial regions. 
Fig. 1 shows, to an exaggerated vertical scale, a 

Fic. 1.—Diagram of continental border-section W. to E. 
portion of a continent seen in section, along with a 
part of a neighbouring ocean, both floating upon the 
basaltic magma. West is to left and east is to right. 
We must imagine that the lower, more viscous part 
of the magma possesses the full west-to-east angular 
velocity of the earth; but the continents and oceans 
and upper layers of the magma are, in virtue of the 
westerly forces just referred to, not moving quite so 
fast in that direction. They respond, in fact, to the 
forces urging them westward. We perceive that this 
involves, of necessity, an east-going force or pressure 
acting upon the submerged parts of the continents, and 
more especially upon the more deeply submerged parts ; 
that is, upon the displacements required by isostasy 
to float the greater raised features of the continents. 
The diagram is intended to illustrate the effect of 
the magmatic pressure with reference to mountain- 
building. We have already seen that mountain ranges 
arise where great depths of sediment collect for long 
ages. These accumulations may amount to several 
miles in depth; the sediments pressing down the 
crust as they collect. 
It is well known that this process creates a linear 
area of weakness in the floating continent. We can 
picture what happens. The great load bends down 
the crust, forcing it deep into the hot magma. It 
becomes seamed with gaping vents and cracks, ex- 
tending parallel with the axis of the trough into which 
the magma forces itself. 
Now, if a horizontal force acts upon a continent 
affected with such an area of weakness, this part yields 
first and the sediments are crushed and forced both 
upwards and downwards. The part that rises up 
forms the mountain range; the part that is thrust | or less melted away during the process. 
NO. 2792, VOL. I11] 

downwards acts as compensation or buoyancy, which 
serves to float the mountain. The one adjusts itself 
to the other. The mountains slowly sink or rise till 
there is equilibrium. Thus, in course of ages, we get the 
floating mountain range. It will be perceived that the 
volume of the downward displacement is much greater 
than the mountain range. This is because the density 
of the crust does not differ greatly from that of the 
sustaining magma. 1 
Such great ranges as the Cordilleras of North and 
South America rose up out of troughs of sediments in 
this manner. They were specially favourably oriented 
to receive the easterly pressure of the underlying 
magma, and, correspondingly, they are in many respects 
the greatest mountain developments of the globe. 
However, while it seems easy to understand that 
the formation of mountain ranges directed more or less 
north and south might arise in this manner, it is more 
difficult to imagine chains of mountains like the Hima- 
layas or like the Pyrenees originating in the west- 
to-east force arising from tidal or precessional effects. 
This brings us to the consideration of the possibility 
of the continents having shifted their relative positions 
during geological time. 
Many are now weighing evidence for and against 
such extraordinary possibilities as to whether the 
Atlantic Ocean is not a comparatively recent innova- 
tion; whether New Zealand was not recently de- 
tached from Australia, and India from the eastern 
shores of Africa, and so on. Before this interesting 
question arose biologists and geologists generally got 
out of their difficulties by assuming the former exist- 
ence of land connexions or “ bridges” which subse- 
quently “ foundered ” and disappeared. 
Now, according to the present explanation of the 
surface movements of the earth, the foundering of such 
“ bridges ’’ would be difficult to realise; for they are 
of lower density than the basaltic magma upon which 
they at one time floated. So that it becomes very diffi- 
cult to imagine the former existence of these bridges. 
Not only is this the case, but also the present theory 
certainly suggests that differential movements of the 
continents might quite possibly have taken place. I 
do not mean to convey that these supposed great 
movements necessarily arise out of our theory, but it 
is at least remarkable that a theory which appears to 
explain much—and on a basis which can claim to be 
more than merely hypothetical—should offer what may 
be regarded as a vera causa for continental drifting if 
other considerations require it. The continents during 
times of revolution become acted upon by forces tending 
to move them towards the east ; and, what is even more 
relevant, these forces must of necessity be different in 
intensity from one continent to another. In fact, 
the magmatic drivesapplied to a continental mass 
depends upon the depth of its immersion and also on 
the existence of great displacements extending down- 
wards into the deeper parts of the magma. 
Another consideration in favour of continental 
drifting must be taken into account. The continents 
become acted upon by these forces only during the 
period of magmatic fluidity. We saw that this fluidity is 
ultimately lost, mainly in consequence of heat escaping 
through the ocean floor ; this floor being probably more 
It may be 
