
May 5, 1923] 
NATURE 
603 

The Surface Movements of the Earth’s Crust. 
By Prof. J. Jory, F.R.S. 
thee land surface of the globe has been, for the 
most part, many times covered by the sea in 
the course of geological time. The mountain ranges 
of the earth, as now known, have only recently at- 
tained their present elevation ; other mountain ranges 
formerly existed which have now been all but obliter- 
ated by the remorseless effects of denudation. 
_ It is important that we should study for a little 
what happens when a great mountain range is developed 
on the surface of the globe. There is a long period 
of preparation for the stately event ; a period many 
millions of years in duration. First, there are signs 
of unrest in the solid land of the continents. The sea 
rises on the coasts and transgresses on the wide lands 
within, very gradually stealing over the lower levels. 
This process may not be steady and continuous. There 
may be periods of retreat followed by periods of 
advance, but always the land, as a whole, goes on sink- 
ing deeper and deeper into the sea. Many millions of 
square miles may be covered with the shallow seas— 
perhaps to a depth of two or more hundred fathoms— 
so that a considerable portion of the land area of the 
globe may become sea before the downward movement 
ceases. This transgression is a slow process ; so slow 
and long-enduring that, while the submergence lasts, 
great depths of sediment accumulate in the trans- 
gressional seas. 
Then at length there comes a resurrection. The 
land begins to emerge ; but not the old land which 
went down. Where the great accumulations of sedi- 
ment had been, mountain ranges arise. In short, 
what arises from the ocean grave is a crushed and 
wrinkled world, shattered by faults and over-thrusts 
and exhibiting every evidence of great horizontal 
compression. One attendant of these events is the 
outbreak of volcanoes and floods of lava welling out 
of fissures in the earth’s crust. The latter generally 
appear along western coasts, or to the west of the 
new-born mountain ranges. 
These events draw to a close when the land has 
attained its former elevation, more or less. There 
is then a new era of geological history—a long era of 
organic progress, lasting many millions of years, during 
which minor oscillations of the crust and local deforma- 
tion may occur. This is a period of active denudation. 
The last-born mountains are degraded by denudation, 
and their sediments collected into the great troughs 
or geosynclines, and the sublime but unreasoning 
sequence of events is repeated all over again. 
Such has béen in leisurely repetition the history 
of the earth. Certain world-revolutions are generally 
accepted—although geologists are not all agreed as 
to their number—as comprised in the period of 150 
or 170 million years which the statistics of denuda- 
tion and the record of thorium lead ascribe to the age 
of our era. Four or five world-revolutions appear to 
enter into that time interval. Thus 30 or more 
millions of years may, tentatively, be ascribed to the 
genesis and consummation of a world-revolution. 
© From a public lecture delivered under the auspices of the al Dublin 
Secisty on March 7. hata 
NO. 2792, VOL. 111] 
From these broad features of geological history it 
is evident that some source of unrest, acting upon the 
surface of the earth, which periodically recuperates its 
strength, runs a course involving an enormous ex- 
penditure of energy, and then dies down into quietude, 
must exist. What can this source of unrest be ? 
In the science of isostasy we are confronted with the 
strange fact (for fact it undoubtedly is), that the lands 
of the earth—firm as they may appear—are yet float- 
ing like rafts or pontoons on a yielding substance far 
beneath. Now, the continents are built of rocks, 
such as granite, gneiss, sandstone, etc., and in the 
same way as the sea-water must be denser than the 
icebergs which float upon it, so the substance which 
buoys up the continents must be denser than granite 
and chemically similar materials. 
We get a very sure guidance as to the nature of the 
sustaining substance in a direct and simple way by 
paying attention to the nature of the lava which is 
poured out in enormous volumes on the surface of 
the land during times of revolution. This substance 
comes up as a thin and very fluent liquid. It may 
flow for 50 or 60 miles over the ground before con- 
gealing. It solidifies to a black and heavy solid—basalt. 
There appears to be no doubt—and in this many 
petrologists are agreed—that basalt is the primary 
rock-magma upon which the continents float and 
which buoys up the great oceans of the earth. Just 
beneath continents and oceans it forms a layer over 
the whole earth—a layer to which isostasy ascribes 
a depth of some 60 to 70 miles. This substance, 
basalt, therefore, plays a very important part in the 
surface history and physical phenomena of the globe. 
Primarily, and most important of all, we know that 
it contains a small quantity of radio-active substances. 
No basalt ever examined failed to reveal this fact. These 
radio-active substances continually evolve heat. We 
know of no conditions which can check, or in any way 
alter or modify, this ceaseless evolution of thermal 
energy. Hence we must recognise that in every 
cubic centimetre of this great magmatic ocean upon 
which the continents and seas float there is a source 
of slow thermal evolution. 
Keeping in mind that the central problem to be 
solved with respect to the great land movements 
affecting the surface of the globe is to account for 
the great outbreak of igneous activity and crustal dis- 
turbance all over the surface of the earth every 25 
or 30 million years, we naturally ask if the perennial 
supply of radio-active heat may not furnish the 
explanation. 
The thermal properties of basalt under ordinary 
conditions have been fairly well examined. At a 
temperature of 1150° it softens, at 1225° it flows 
freely, forming a very mobile but heavy liquid. In 
passing from one state to the other there is a 
volume increase of about ro per cent. of the initial 
volume. This may be a rather excessive value. It 
is not less than 6 per cent. 
Now, the fact that the basalt in these great floods 
reached the surface in a fluid state is adequate proof 
