Address to Section C, Geology. 417 
temperature of only 1,500° C. Whether the further conclusion that 
this heat is due to the action of the radium in the crust be established 
or not, it is gratifying to hear a physicist arguing in favour of a 
moderate and uniform internal temperature. 
All that the actual observations prove and that geological theories 
require is that the material within the earth be intensely hot, and 
that it lie under such overwhelming pressure that it would as readily 
change its form and as quickly fill up an accessible cavity as any 
liquid would do. Whether such a condition is to be described as 
solid, liquid, or gaseous is of little concern to geologists. 
Ill. Zhe Deep-seated Control over the Earth's Surface.—The modern 
view of the structure of the earth adds greatly to the interest of its 
study, for it recognises the world as an individual entity of which 
both the geological structure and the history have to be considered as 
a whole. Once the earth was regarded as a mere lifeless, inert mass 
which has been spun by the force of gravity, that hurls it on its 
course into the shape of a simple oblate spheroid. Corresponding 
with this astronomical teaching as to the shape of the world was 
the geological doctrine that all its topography is the work of local 
geographical agents, whose control over the surface of the earth is as 
absolute as that of the sculptor’s chisel over a block of marble. 
Both these conceptions are now only of historic interest. The 
irregular individual shape of the earth is expressed by its description 
as a geoid. The processes which have produced its varying shape 
haye also controlled its geological history and evolution, for they 
cause disturbances of the crust, which affect the whole earth 
simultaneously ; and so the geographical agents are given similar 
work and powers at the same time in different places. 
Hence there is a remarkable worldwide uniformity in the general 
characters of the sedimentary deposits of each of the geological 
systems. The last pre-Cambrian system includes thick masses of 
felspathic sandstones alike in the Torridonian of Scotland, the 
sparagmite of Scandinavia, the Keweenawan Sandstones of the United 
States, and perhaps also the quartzites of the Rand. The Cambrian 
has its greywackes and coarse slates and its numerous phosphatic 
limestones; the Ordovician its prevalent shales and slates; the 
Silurian its episodal limestones and shales. The Devonian has its 
wide areas of Old Red Sandstones as a continental type, while its 
marine representatives show the prevalence of coarse grits and sand- 
stones in the lower series, of limestones and slates in the middle series, 
and the recurrence of sandstones in the upper series; and this sequence 
occurs alike in North-Western Europe, in America, and Australia. 
The Carboniferous contains the first regional beds of thick limestone 
and the first important Coal-measures. The Trias is as characterised 
by rocks indicating arid continental conditions in America and 
Australia and South Africa, as Professor Watts has shown then 
prevailed in the neighbourhood of Leicester. In the Mesozoic era 
we owe to Suess the demonstration of the worldwide influence of 
those marine encroachments or ‘transgressions’ whereby the great 
continents of the Trias were gradually submerged by the rising sea. 
Speaking generally, there is a remarkable lithological resemblance 
DECADE Y.—YOL. IV.—wNO. IX. 27 
