Address by Prof. Sir T. H. Holland. 413 



Since the days of Laplace all naturalists have been forced to accept the idea of 

 a solar system formed by the cooling and condensation of a spheroidal gaseous 

 nebula ; and all except those geologists who have vainly searched for traces 

 of the primeval crust have been happy in this belief. 



Recently, however, Dr. F. R. Moulton and Professor T. C. Chamberlin 

 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 tiying off into space. With the growth of the young globe, the 

 compression towai'ds 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 fonned, and became 

 charged with water, when the now familiar processes of weathering, erosion, 

 and deposition produced the film of ' rust ' which geologists know as sedi- 

 mentary 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 radio-active 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 inconspicuous 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 thi'oughout, 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 



^ J. Joly, Radioactivity and Geology, 1909, pp. 168-72. 

 - E. H. L. Schwarz, Causal Geology, 1910. 



