346 TRANSACTIONS OF SECTION ©. 
in America have brought together arguments from different points of view to 
construct the solar system by the aggregation of innumerable small bodies, 
‘planetesimals,’ which have gathered into knots to form the planets. Thus, 
the Earth is supposed to have grown gradually by the accretion of meteoritic 
matter, and even now, although the process has nearly ceased, it receives much 
meteoritic material from outside. 
With the Chamberlin-Moulton theory there must have been a time when the 
gravity of the Earth was insufficient to hold an atmosphere of any but the 
heavier gases, such as carbon dioxide; later, the Earth became heavy enough 
to retain oxygen, then nitrogen, water-vapour, and helium; while even now it 
may not be sufficiently attractive to prevent the light and agile molecule of 
hydrogen from flying off into space. With the growth of the young globe, the 
compression towards the centre produced heat enough to melt the accumulated 
fragments of meteoritic matter, and the molten material thus formed welled 
out at the surface. Such volcanic action is supposed to have predominated 
at the surface until an appreciable atmosphere was formed, and became charged 
with water, when the now familiar processes of weathering, erosion, and deposi- 
tion produced the film of ‘rust ’ which geologists know as sedimentary rocks. 
With this last addition to the variegated array of theories about the physical 
condition of the Earth and about its genealogy, the scientific world began again 
to settle down into serenity, comforted by the happy feeling that all at any rate 
agree in regarding the Earth as a gradually cooling body, with many millions 
of years still before it. Then came the discovery of radium, and, with it at 
first, an assurance that geologists were justified in claiming a long past, to be 
followed by a longer future than the most optimistic philosopher had dared 
before to assume with our apparently limited store of Earth-heat. Now, however, 
Professor Joly warns us that if the deeper parts of the globe contain anything 
near the proportion of radioactive bodies found by him in the superficial rocks, 
we may even be tending in the other direction; that, instead of a peaceful 
cooling, our descendants may have to face a catastrophic heating; the now in- 
conspicuous little body known as the Earth may indeed yet become famous 
through the Universe as a new star.* 
To add to the variety of ideas regarding the present state of the Earth’s 
interior, Professor Schwarz, of Grahamstown,* concludes that our volcanic 
phenomena can be accounted for on the assumption that the main mass of the 
Earth below a superficial layer is cold and solid throughout, being composed, 
like the meteorites, largely of unaltered ferromagnesian silicates and iron, 
Thus, we see, whole fleets of hypotheses have been launched on this sea of 
controversy : some of the craft have been decoyed by the cipher-signals of the 
mathematician; some have foundered after bombardment by the heavy missiles 
classically reserved for use by militant geologists; others, though built in the 
dockyard of physicists, have suffered from the spontaneous combustion set up 
by an inadvertent shipment of radium. Still, some of these hypotheses are yet 
apparently seaworthy, and it may not be unprofitable to compare them with 
recently acquired data. 
The nearest approach to actual observation with regard to the state of the 
Earth’s interior has been obtained by the seismograph, designed to record the 
movements of seismic waves at great distances from the disturbing earthquake. 
Some of the waves sent forth from an earthquake-centre travel through the 
Earth, and some travel around by the superficial crust, the former reaching 
the distant seismograph before the latter. The seismograph, by its record of the 
wayes that travel through the Earth, has thus given a certain amount of infor- 
mation regarding the state of the Earth’s interior which R. D. Oldham aptly 
regards as analogous to that given by the spectroscope “ with regard to the 
inaccessible atmosphere of the Sun. 
2 J. Joly, Radioactivity and Geology, 1909, pp. 168-172. 
3. H. L. Schwarz, Causal Geology, 1910. 
4 In his Presidential Address to the Geological Society of London in 1909, 
Professor W. J. Sollas (Proc. Geol. Soc., 1909, p. Ixxxvii) credits H. Benndorf 
(Mitth. Geol. Gesellsch. Wien, I., 1908, 336) with this pretty analogy, but 
Oldham has the precedence by just two years (cf. Quart. Journ. Geol. Soc., 
vol. 62, 1906, p. 456). 
