54 



NATURE 



[Nov. 2 1, 1889 



The Physics of the Sub-oceanic Crust. 



In the new edition of his "Physics of the Earth's Crust," 

 Mr. Fisher has made a great advance on his former position, for 

 he sees his way to explain the formation of mountain chains, and 

 all the phenomena of compression which are so strikingly 

 exhibited in the crust of the earth, without depending on his 

 former theory of columnar expansion, and without falling back on 

 the contraction hypothesis. 



He believes that the existence of a liquid substratum beneath 

 a thin crust is consistent with the physical conditions of the 

 universe ; and argues that no appreciable tide would be produced 

 in it if the liquid magma consisted of an intimate association of 

 fused rock and dissolved gases. He further concludes that this 

 magma is not an inert or motionless liquid, but one in which 

 convection currents are constantly bringing up heat from below, 

 and leading to frequent internal displacements of mass. 



In this hypothesis he finds a means of explaining the move- 

 ments of the earth's crust. Whether Mr. Fisher's position can 

 be maintained must be decided by those who are accustomed to 

 deal with the physical problems involved, but geologists will be 

 glad if it should prove that the objections to the existence of a 

 liquid substratum have been succes'-fully met, for they have 

 always found a difficulty in explaining geological phenomena 

 without having recourse to the supposition of a liquid layer. 



One of the most important chapters in the book is that on the 

 sub-oceanic crust, and it is on this that I propose to offer a few 

 remarks, taking it for granted that a truly liquid substratum with 

 a play of convection currents does really exist. 



Mr. Fisher's object is to ascertain the thickness and density of 

 those parts of the crust which lie beneath the oceans, and to see 

 whether in these respects they diflFer from the continental 

 portions. This he does by making a series of assumptions, and 

 considering how far the results are compatible with known facts 

 and conditions. This process involves the dismissal of certain 

 hypotheses, but although he eventually finds one which fulfils the 

 requisite conditions, it does not follow that no other equally 

 satisfactory hypothesis can be found. Consequently his results 

 though interesting cannot be regarded as final. The suppositions 

 he is obliged to introduce before obtaining satisfactory results 

 are, that the density of the subtratum beneath the continental 

 and the sub-oceanic portions of the crust is different, and that the 

 sub-oceanic crust consists of two layers of different densities. 



It is conceivable, however, that the lower part of the crust is 

 everywhere denser than the upper part, and consequently that two 

 layers of continental crust should be introduced into the problem ; 

 whether this hypothesis would likewise fulfil the conditions, and 

 whether it would lead to the same results as that which Mr. 

 Fisher adopts, could only be ascertained by trial. Mr. Fisher 

 informs me that he has not made this trial, and that every 

 additional assumption introduced increases the great labour of 

 the calculations. 



Let us assume, however, that no other hypothesis would satisfy 

 the conditions so well as that which he has adopted, and let us 

 see to what conclusions it leads. Mr. Fisher derives from it the 

 following important results : — 



(i) That the sub-oceanic crust dips more deeply into the 

 substratum than the continental crust. 



(2) That its lower part is more dense than the substratum. 



(3) That the density of the liquid substratum is less beneath 

 the oceans than beneath the continents. 



This last result leads to the conclusion that the differences of 

 density in the substratum must give rise to ascending and descend- 

 ing convection currents, and that the ascending currents will rise 

 beneath the oceans while the descending currents will occur 

 beneath the continents. "That the former occupy so much 

 larger an area is," he says, "no more than we might expect, 

 because to whatever immediate cause they may be due, they are 

 ultimately the result of secular cooling. . . . The descending 

 being merely return currents will be confined to the smaller area, 

 but on that account they will move the more rapidly." 



Finally he says that these conclusions confirm the theory of the 

 permanence of oceans, "because it is difficult to conceive how 

 the subjacent crust, once more dense, can have subsequently 

 passed into the less dense condition which would be requisite to 

 render it continental." I venture to think he is hardly justified 

 in making this unqualified statement, and purpose to show that 

 his results only confirm the theory of the permanence of oceans 

 in a limited and partial manner. 



In the first place, if chapters xvii. and xxiv. are read carefully, 



it will be obvious that Mr. Fisher uses the terms oceanic and sub- 

 oceanic in a special sense. On p. 233 he classes areas having less 

 than two vertical miles of water as " extensions of the elevations 

 that produced the continents," and even those with depths of two 

 to three miles of water he regards as "sometimes connected with 

 and prolongations of the first." In other words, he looks upon 

 the shallower parts of the great oceans from a continental coast- 

 line to a depth of at least 2000 fathoms as extensions of the 

 continental elevations. 



Again, on p. 331 we find him saying that New Caledonia and 

 the Seychelles are not properly speaking oceanic islands, because 

 the first is a prolongation of the submerged ridge which connects 

 New Zealand with North Australia, and because the latter 

 belongs to an extension of the Madagascar ridge into the Indian 

 Ocean. Now a reference to the physical chart of the oceans 

 given in the "Narrative of the Cruise of the Challenger" 

 (vol. i.) shows that the looo-fathom line completely encircles 

 New Caledonia and the adjacent islands, and that the submerged 

 ridge which he speaks of would be a very narrow one unless we 

 regard it as extending to the line of 2000 fathoms ; but this line 

 includes also the Solomon Islands, the Fijis, and the Friendy 

 Islands, so that if New Caledonia cannot be considered as an 

 oceanic island neither can the other islands just mentioned, 

 though no one would reject them from that category on other 

 grounds. Similarly, the Seychelles and Amirantes are surrounded 

 by water of more than 1000 fathoms, and are usually regarded as 

 oceanic islands. The same may be said of Barbados, where 

 stratified Neozoic rocks are found. 



The contour-line of 1000 fathoms has, I think, been generally 

 taken by recent writers as the approximate limit of the 

 continental elevations, the space outside this being regarded as 

 oceanic ; the islands which rise from depths of over 1000 

 fathoms would on this view be necessarily classed as oceanic, 

 and as a matter of fact all such islands come within the terms of 

 Sir A. Wallace's definition of an oceanic island except that a 

 few of them are not entirely of volcanic or coralline composition. 

 To exclude all the islands which rise from within the 2000-fathom 

 limit would necessitate the division of oceanic islands into two 

 classes, the definition of which would be difficult. 



I am not saying that such a distinction would be incorrect, or 

 that Mr. Fisher has no right to assign larger limits to the 

 continental elevations and narrower limits to the oceans : I only 

 desire to show that he takes a special view, and that he declines 

 to regard islands which rise from less than 2000 fathoms as 

 specimens of the sub-oceanic crust. His discussion of the 

 probable structure of the sub-oceanic crust deals therefore with 

 areas which are covered by water of three miles or more in depth — 

 that is to say, from about 2500 to 5000 fathoms, and the 

 comparison which he makes between patches of sub-continental 

 and sub- oceanic crust is really between a piece of continental 

 land and a piece below an area of deep ocean at a considerable 

 distance from the continents. 



With regard to this point, I have had the advantage of a 

 further explanation from Mr. Fisher ; writing to me he says : — 

 " My sub-oceanic patch may be anywhere under the ocean, but 

 you must remember that all the quantities are subject to change 

 except c, p, fM, ff, as 5 diminishes ; i.e. as the ocean grows 

 shallower toward the coast-lines, the thicknesses and densities 

 merge into those at the sea-level, the second layer of the sub- 

 oceanic crust at the same time thinning away to nothing. You 

 are quite right in thinking that in a general way in discussing 

 the sub-oceanic crust I am dealing with the crust at a consider- 

 able distance from the continents I do not profess to 



explain the structure of the crust of the earth in those parts which 

 appear to have sometimes been land and sometimes sea. I 

 should, however, guess that having been at times land the 

 crust there resembles the present continental crust. Still the 

 equations (p. 242) must apply to these parts if only we knew 

 what assumptions to make." 



Since, therefore, there are regions of sub-oceanic crust the 

 structure of which may resemble that of the continental crust 

 rather than that beneath the central parts of the oceans, it is 

 clearly of importance to consider the position and extent of these 

 regions. Let us first take that part of the Pacific Ocean in which 

 New Caledonia is situate ; if we are to regard it as a submerged 

 plateau which may once have been continental land, it acquires 

 a special interest. The contour of 2000 fathoms which unites 

 New Caledonia to Australia and New Zealand extends from the 

 north coast of New Guinea by the Solomon Islands to Samoa, 

 and then bends southward to New Zealand, but curves out again 



