606 
NATURE 
[May 5, 1923 

that the reduction in thickness of the ocean floor is 
carried so far as to remove what is really the main 
obstacle to differential continental movement—the 
existence of a strong and rigid ocean floor, holding the 
continents immovably fixed to one another. 
We return for a moment to the problem of the 
elevation of such ranges as the Himalayas, which 
trend more or less east and west. We are now pre- 
pared for the possibility that the explanation of these 
events was due to a certain small amount of con- 
tinental movement. It is a fact that tidal and pre- 
cessional forces are greatest in equatorial regions. 
May it not have been that the great continent of Africa, 
experiencing the effects of this, rotated just a little, 
its southern extremity moving eastward; and so also 
for Peninsular India ; so also for the Spanish Penin- 
sula? Asmall turning movement, crushing the ancient 
geosynclines, would suffice. For, after all, the greatest 
mountains are but very tiny wrinkles upon the surface 
of this huge world. 
The outflows of lava on the western coasts of the 
continents, or to the west of great mountain masses, 
or brought up by the downward faulting of rift valleys, 
to which I have already referred, seem to give us direct 
evidence of the magmatic pressures of which we have 
been speaking. The injection of lava inte the great 
mountains, or its ejection from lofty volcanoes, finds 
explanation in the great volumes of included basaltic 
magma which are taken up in the crushed and shattered 
sediments of the geosynclines when these are floated 
up from the depths of the earth’s crust. 
In the foregoing remarks I have endeavoured to 
trace, on the basis of isostasy and radio-activity, the 
existence of cyclical changes, prevailing in the isostatic 
layer, which are in harmony with the observed recur; 
rent world-revolutions. While a certain grand sim- 
plicity in the nature of these events, and the existence 
of a general resemblance between the character of one 
revolution and that of the next, permit of this treatment, 
it would be an erroneous inference that the physical 
events of historical geology are concentrated in the 
relatively brief periods of world-wide mountain-build- 
ing. For in truth an endless succession of minor 
changes have affected the crust of the earth. Between 
the great revolutions transgressions of the ocean have 
occurred over considerable areas. Crustal warping, 
and even mountain elevation of lesser ranges and 
batholithic invasion of the crust, as well as renewed 
volcanism, have not been uncommon. All the events 
of the greater revolutions may appear locally, and 
always on a lesser scale. 
In point of fact, these lesser, inter-revolutionary 
events are, probably, part of the primary phenomenon 
and owe their existence to energy concerned with the 
genesis of the former. For consider that during millions 
of years the continental crust, throughout every part 
of it, has been subjected to those same enormous 
stresses, vertical and horizontal, that served to uplift the 
Cordilleras to heights of more than 20,000 feet; and 
that, at the time when the floor of the ocean congealed 
around the continents and tidal effects died out, the 
vast volume of the land was left deformed by these 
great stresses, strained, often to fracture wherever 
rigidity prevailed, and with isostatic adjustments’ pro- 
foundly disturbed. 
NO. 2792, VOL. 111] 

The inter-revolutionary periods of geological history 
must witness the readjustments necessitated by this 
accumulation of potential energy. Areas of low 
resistance—z.e. the geosynclines, the volcanic areas, 
or recently deformed regions—must experience the 
concentrated results. Moreover, all the conditions 
for very prolonged continuance of these minor 
activities exist. For there is no other way in which 
the accumulated energy may find relief save in crustal 
disturbance or readjustment. It will be slowly doled 
out for ages as the effects of denudation call upon it, 
or as thermal events give it occasion to intervene, for the 
cooling of the magma beneath the continents must be 
extremely slow. Sheets of melted lava must underlie 
them, throughout almost the whole of geological 
time, although deeper down there may be comparative 
rigidity. 
It will be apparent from all this that there is nothing 
unaccountable either in the existence or nature of 
inter-revolutionary events. On the contrary, we may 
say that their absence would be highly unaccountable. 
Even more, I think that as we study these events we 
must conclude that they cannot represent more than 
a fraction of the stored energy attending a great. — 
revolution. 
This leads to the energy question at large. Whence 
does it allcome ? To answer fully that question would 
lead us back over much of the ground we have already 
pursued. But as regards energy other than radio- 
active we may briefly answer: ‘“‘ From the rotation 
of the earth.” And is it not adequate? Look at the 
diagram of an earth-sector (Fig. 2) ; with a floating 
Fic. 2.—Earth-sector showing basaltic layer and continents to 
scale of radius. 
crust 20 miles thick and an isostatic layer 7o miles 
deep. Consider how petty are the crustal energies 
contrasted with the stored energy of the globe, built 
as it is out of materials twice as dense as the conti- 
nental rocks, and possessed, even to-day, of a surface 
velocity of 1000 miles an hour. 
In its biological aspect how great and wonderful it 
all is! The living being working out his destiny on 
this poor raft, unknowing of the fiery ocean upon 
which his world is floating : unknowing of the inevit- 
able sinking and uplifting which in truth largely 
control the destinies of his race. Death-dealing forces 
all around, and yet the light of life shining age after 
age upon the earth. } 

