416 Notices of Memoirs — Australia, 1914 — 



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 Columbia to Europe must have travelled under the 

 broadest and deepest part of the North Atlantic basin, while 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 reliable 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 Geological Section of the British 

 Association at Dover in 1899, the late Sir John Murray drew 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 processes the more siliceous and specifically lighter constituents are left 

 behind on the continents, while the heavier bases are carried out to the ocean. 

 It is to this process that Professor T. C. Chamberlin^ also ascribes the origin 

 of the depressions in which the oceanic waters have accumulated. As 

 a corollary of the planetesimal theory, Chamberlin assumes that water began to 

 be forced out of the porous surface blocks of the accumulated meteoritic material 

 when the Earth's radius was between 1,500 and 1,800 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 proceeded by the 

 infall of planetesimals, and while those meteorites 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 the continents. According to the 

 deduction suggested by Oldham from the seismographic records, there is 

 a noticeable difference in the sub-oceanic areas to depths of between 1,000 and 

 1,300 miles — a layer in which the followers of Chamberlin's theory might 

 reasonably expect some physical expression of the partially 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 accepted by Pratt for 

 his theory of compensation, as well as by Button as a wide expression of the 

 theory of isostasy. Chamberlin^ thus explains the general prevalence of basic 

 lavas in oceanic volcanoes. 



The apparent heterogeneity indicated in the outer shell of the Earth to 

 depths of 1,000 miles is naturally in conflict with the assumption that from 

 30 miles or so down the materials are in a liquid condition ; at any rate, the 

 idea conflicts with Fisher's extreme conception of the liquid substratum, 

 in which the fluidity is supposed to be sufficient for the production of 

 convection currents, upwards beneath the oceanic depressions, spreading 

 horizontally towards the continents, and thence downwards to complete the 

 circuit. 



The idea that changes of azimuth and of latitude may be brought about by 

 the sliding of the Earth's crust over its core has been put forward more than 



^ Chamberlin and Salisbury, Geology, vol. ii, pp. 106-11, 1906. 

 ^ Geology, vol. ii, p. 120, 1906. 



