604 
that it was at a high temperature in the regions deep 
down from which it came. This is its condition 
generally all the world over during times of revolu- 
tion. There are many reasons for believing that at 
the present time it does not and cannot generally 
exist in the fluid state ; although deep pockets of the 
fluid magma must probably exist at all times through- 
out the magma-ocean and beneath the continents, 
there extends for a very long period after a revolu- 
tion a shallow layer of the melted rock. Generally 
throughout the deep isostatic layer it possesses the 
characters of a plastic solid and is yielding enough 
for the continents to float upon it. The addition of 
a certain known quantity of heat to each gram of the 
highly heated basalt will convert it to the liquid state. 
We know, as the result of many experiments, the 
quantity of radio-active substances in basalt. Samples 
from various great lava flows and volcanoes have been 
examined. ‘There are certain variations in the quanti- 
ties observed from one great flow to another. Taking 
a mean we can calculate the quantity of heat which 
would be generated, say, in one million years in each 
gram of the basalt. Briefly stated, the results of 
our investigation show that the heat accumulated in 
about 25 million years would suffice to turn the solid 
basalt, nearly at its melting-point, into a liquid. 
The first effect of this change will be a considerable 
expansion in volume and corresponding loss of 
density and buoyancy. 
solid basalt near its melting-point expands some 6 
to 10 per cent. of its volume in changing to the liquid 
state. The result upon the continents is easily, in- 
ferred. When a ship sails from the salt water of the 
ocean into a river of fresh water it sinks a little; so 
also the continents will sink a little. The waters of 
the ocean will therefore transgress upon the lands, 
advancing century after century as the basalt changes 
its state, as we know happens in periods preceding 
a revolution. Hence the earliest phase of geological 
change finds an explanation in the melting of the 
basalt which floats the continents. 
But other consecutive consequences follow. For 
when, all over the earth, beneath continents and 
oceans, there extends a deep sea of melted lava, it is 
evident that conditions arise favourable to greatly 
increased volcanism both on the land and over the 
floor of the oceans. 
The melted basalt will again lose heat and revert 
to the solid state. It may take 3 to 4 million years for 
this to happen, but happen it must. For liquids part 
with heat much more quickly than solids, just because 
circulation can go on in them. Now the basalt, where 
it laps against the rocky floor underlying the oceans, 
loses its heat far more rapidly than radio-activity can 
supply it. It probably melts away a good deal of 
the ocean floor in the process of parting with its heat. 
The ocean floor is very probably, almost certainly, 
also basalt. Possibly this floor becomes very thin 
indeed in the course of the long period during which 
the great ocean of lava is returning to the solid or 
plastic state. 
It will be understood that the change of state has 
completely altered the conditions of heat-loss, the 
gain of heat per gram remaining the same at all times. 
The solid basalt can only lose heat by conductivity— 
NO. 2792, VOL. 111] 
NATURE 
For, as has been stated, the | 

[May 5, 1923 

a very slow process. Beneath the continents even 
this means of escape is almost closed, because the base 
of the continents possesses a high temperature, arising 
from the radio-active content of the continental 
materials themselves. Beneath the ocean, a few 
miles down, the conditions become much the same. 
Thus the solidified magma must conserve practically 
all its heat-gains. When fusion becomes general con- 
vection begins, as well as other movements later to be 
referred to. The escape of heat beneath the oceans 
becomes then relatively rapid. 
But now notice the effect upon the continents of 
this reversion to the solid condition. When the basalt 
regains the solid state it also regains its original density ; 
and the Jand regains its original buoyancy. The 
continents must now rise again to their former altitude 
above the sea. They are as ships passing from the 
river to the ocean. The waters which flowed in upon 
the continents during the slow process of the melting 
of the basalt must recede again as the basalt re- 
solidifies. Hence a final great phase of geologic change 
finds explanation in the physical properties of the 
basaltic ocean and its inevitable thermal changes. 
We can only discuss with any degree of definiteness 
the events progressing in the upper region of the great 
basaltic ocean. For the depth of this ocean is prob- 
ably not less than 60 miles, and the pressures pre- 
vailing in such depths greatly modify the behaviour 
of substances experiencing accession or loss of heat, 
but there is no reason to believe that any effects to 
which reference has been made will be seriously modified. 
On the contrary, the effects, so far as we can infer 
them, of great pressure in the depths appear to bring 
events still more into harmony with geological observa- 
tions and inferences. 
From what has been stated we see that the reason 
for the long time intervals between the epochs of world- 
wide revolution is to be found in the smallness of the 
quantity of radio-active substances existing in the great 
sustaining magma supporting the continents and the 
oceans. On an average, it takes some 25 millions 
of years for the change of state to be brought about 
attending which the continents must sink and the 
waters transgress upon their surface. Then some 3 
to 5 millions of years may be required for the stored 
radio-active heat to be again dissipated. The cycle is 
therefore accomplished in, say, 30 millions of years. 
These figures are given merely as suggestive of what 
might prevail. Various causes, which cannot be dis- 
cussed, may modify them. 
We live at a period immediately succeeding a very 
great world-revolution. The lava ocean has lost its 
heat of fluidity for the most part, and the continents 
float upon the basalt sea as upon a plastic or viscous 
body nearly at its melting temperature. These con- 
ditions are really very wonderful ; but the explanation 
of our immunity is simple. The melting-point of the 
continental rocks is from 200° to 500° higher than that 
of basalt. Again, solid rock conducts heat badly. 
Hence little or no heat reaches us from the fiery ocean 
beneath. 
We have next to consider if we cannot find an ex- 
planation of mountain-building and volcanic phenomena 
as involved in the changes we have been discussing. 
We know that the ocean tides are due to lunar and 
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