September 3, 1914] 



NATURE 



1 1 



earthquakes show that there is either no, or at most a 

 doubtful, record for the second phase, which is known 

 to be due to the so-called distortional waves. 



Oldham's deductions are based confessedly on a 

 small number of earthquake records — he considered 

 fourteen examples only — but the conclusions based on 

 a small number of trustworthy records, from which 

 variations due to the different methods of marking the 

 phases are eliminated, are more trustworthy than those 

 for which there are imperfect distant records as well as 

 doubts regarding the exact times of the disturbances. 

 If these observations, however, be confirmed by further 

 records, we are justified in assuming that below the 

 heterogeneous crust there is a thick shell of elastic 

 material, fairly homogeneous to about six-tenths of 

 the radius, surrounding a central core, four-tenths in 

 thickness, which possesses physical properties utterly 

 unlike those of the outer layers ; for in this core the 

 "distortional" waves ar.e either damped completely or 

 are transmitted at very much lower speeds than in the 

 shell. 



One cannot consider this interesting inference from 

 the seismographic data without being reminded of the 

 contention of Ritter, Arrhenius, and Wilde regarding 

 the possibility of a persisteni. gaseous core still above 

 the critical temperature of the substances of which it 

 is composed. According to Ritter,* the gaseous core 

 is surrounded by a solid shell. Dr. Wilde * postulates 

 the existence of a liquid substratum and a gaseous 

 core within a solid crust, the two outer shells having 

 a thickness that is "not very considerable." Arrhenius 

 assumes from purely physical considerations that the 

 solid crust is only about twenty-five miles thick, that 

 below this it is possibly in a molten condition for 

 about a hundred and fifty miles, and that the rest is 

 a gas largely composed of iron under a pressure so 

 great that its compressibility is not much less than 

 that of steel. 



The whole of these conclusions, being based on 

 assumptions regarding the physical properties of 

 matter under conditions of temperature and pressure 

 that are well beyond those of actual experience, must 

 be put on a plane of science well below that occupied 

 by the investigations initiated by Oldham, who opens 

 up a line of research in which, as said before, the 

 seismograph may justifiably be compared with the 

 spectroscope as an instrument for observing some in- 

 accessible regions of nature. 



The mathematician apparently finds it just as easy 

 to prove that the earth is solid throughout as to show 

 by extrapolation from known physical values that it 

 must be largely gaseous. As Huxley said in his 

 presidential address to the Geological Society in 1869. 

 the mathematical mill is a mill which grinds you stuff 

 of any degree of fineness, but, nevertheless, it can 

 grind only what is put into it ; and the seismograph 

 thus offers a new source of substantial grist. Now 

 that it is fairly certain that some of the earthquake- 

 waves pass through the deeper parts of the earth, it 

 is obvious thai a fruitful development of science will 

 follow successful efforts to introduce precision in 

 recording, and uniformity of expression in reading, 

 seismographic records. 



Oldham ^^ has pointed out another way in which 



* A. Ritter, " Untersiichuneen uber die Hohe der Atmosphare und die 

 'Constitution gasformiger Weltkorper," Wiedemann's Ann. d. Phys. und 

 Chem., vol. V. 405, 543(1878); vol. vi. 135 (1879); vii. 304 (1S79); vol. viii. 

 157(1879). _ 



" " On the Causes of the Phenomena of Terrestrial Magnetism, Pam- 

 pMet, 1890. p. 2. The idea that the Earth's magnetism is due to the elec- 

 tricity generated bv the friction between the shell and the core, mating 

 with a different motion, was suffgested bv Dr. Wi'de in 1902 (Mem. Manch. 

 Lit. and Phil. Soc, vol. xlvi., part iv. p. 8, 1902). A-imilar suggestion 

 based also on Halley's conception of a separately rotating inner core was 

 made previously by .Sir F. J. Evans in 1878 (" Remarkable Changes in the 

 Earth's Magnetism," Nature, vol. xviii. p. 80). 



'* Quart. Journ. Geol. Soc , vol. Ix'ii, ^44-350 (1907). 



analysis of seismographic records may lead to informa- 

 tion regarding intra-telluric conditions by comparing 

 the records of waves that pass under the oceanic 

 depressions with those that are sub-continental for 

 the whole or most of their paths. By comparing tht- 

 records in Europe of the Colombian earthquake of 

 January 31, 1906, with those of the San Francisco 

 earthquake in the following April, there was a greater 

 interval noticed between the first and second phases 

 of the Californian earthquake — an interval greater than 

 can be accounted for by mere difference of distance 

 between the origin of the shock and the recording 

 instruments. The seismic waves which passed from 

 Colombia to Europe must have travelled under the 

 broadest and deepest part of the North Atlantic basin, 

 whilst those from California ran under the continent 

 of North America, crossed the North Atlantic not far 

 south of Iceland, and approached Europe from the 

 north-west, the wave paths throughout being under 

 continents or the continental shelf of the North Atlantic. 

 There is thus suggested some difference between the 

 elastic conditions of the sub-oceanic and the sub- 

 continental parts of the crust — a difference which, 

 judging by the particular instances discussed, may 

 extend to a depth of one-quarter of the radius, but is 

 not noticeable in the waves which penetrate to one- 

 third of the radius below the surface. 



Obviously these data must be multiplied many times 

 before they can be regarded as a trustworthy index to a 

 natural law ; but it is significant that this indication 

 of a difference between the physical nature of the sub- 

 oceanic and sub-continental parts of the crust is in 

 rough correspondence with the conclusions previously 

 suggested on quite other grounds. 



In his presidential address to the Geographical Sec- 

 tion of the British Association at Dover in 1899, the 

 late Sir John Murray directed attention to the chemical 

 differentiation which has been going on between the 

 continents and the oceans since the processes of 

 weathering and denudation commenced. By these pro- 

 cesses the more siliceous and specifically lighter con- 

 stituents are left behind on the continents, while the 

 heavier bases are carried out to the ocean. It is to this 

 process that Prof. T. C. Chamberlin " also ascribes 

 the origin of the depressions in which the oceanic waters 

 have accumulated. As a corollar>' of the planete>imal 

 theorv, Chamberlin assumes that water began to be 

 forced out of the porous surface blocks of the accumu- 

 lated meteoritic material when the earth's radius was 

 between 1500 and 1800 miles shorter than it is now; 

 at that time pools of water began to be formed on 

 the surface, and the atmosphere, just commencing its 

 work, began the operation of leaching the heavier 

 bases out of the highlands. Growth of the world pro- 

 ceeded bv the infall of planetesimals, and while thoso 

 meteoriti'es that fell on the highlands became deprived 

 of their soluble bases, those that fell into the young 

 ocean were merely buried unaltered. Thus, by the 

 time the earth reached its present size its crust under 

 the oceanic depressions must have developed a 

 chemical composition differing from that under th<=' 

 continents. According to the deduction suggested bv 

 Oldham from the seismographic records, there is a 

 noticeable difference in the sub-oceanic areas to 

 depths of between 1000 and 1:^00 miles — a layer in 

 which the followers of Chamberlin 's theorv- might 

 reasonably expect some phvsical expression of the 

 partiallv developed chemical differentiation. 



The occurrence of denser material below the ocean? 

 has, of course, long been assumed from the deflection 

 of the plumb-line, and was accented by Pratt for his 

 theorv of compensation, as well as by Dutton as a 

 wide expression of the theory of isostasy. Cham- 



n Chamberlin and Salisbury, " GeoI«»gy," vol. ii. 1906, 106-111. 



XO. 2340, VOL. 94] 



