Nov. 2 1, 1889] 



NATURE 



55 



... :j include the Chatham and Antipodes Islands, some 600 

 miles to the south-east of New Zealand. Southward it has a 

 connection with the Antarctic continent, but a deep gulf of over 

 20CO fathoms runs far up outside the east coast of Australia. 

 The area within the 2000-falhom line measures about 2500 

 miles across its northern portion, and has an extreme length of 

 ibout 3600 miles from its northern border to the south end of 

 New Zealand. 



If this large area is not to be regarded as strictly oceanic — that 

 is to say, if the physical structure of the crust beneath it differs 

 from that of the crust beneath the deeper ocean outside it — and 

 if its geological history is different from that of this deeper 

 oceanic area, and is comparable with that of a continent, then a 

 very important modification is introduced into the theory of the 

 jermanence of oceans and continents. 



We learn that an area now covered with oceanic deposits may 

 not have been always ocean, and this is precisely what Lyell and 

 liis followers have always maintained ; for if so large a part of 

 I he Pacific may have been land (say in the Cretaceous period), 

 there has been what most geologists would consider to be a 

 change from continental to oceanic conditions ; and if, being such 

 a transmutable region, it may eventually be raised again till 

 large parts of it become land surfaces, round which shallow 

 water deposits could be formed, it would exhibit strata of deep- 

 sea origin (usually called oceanic) intercalated between forma- 

 tions of the ordinary continental type. 



Another region where similar transmutations appear to have 

 taken place is that of the West Indian Islands with the adjoining 

 area of the Caribbean Sea and a portion of the Western Atlantic. 

 Of this region the structure of Barbados is an illustration. 

 That island conforms to the ordinary definition of an oceanic 

 island ; it is separated from South America and the rest of the 

 Antilles by water of over 1000 fathoms, and the scanty fauna 

 •which it possesses is not such as would have been introduced by any 

 former land connection. Its geological structure is simple but 

 striking : there are no volcanic rocks, but a basal series of 

 sandstones and clays that are similar to the older Tertiaries of 

 Trinidad, and may be regarded as testifying to a former northern 

 extension of the South American continent ; above these are 

 oceanic deposits, consolidated radiolarian and foraminiferal oozes, 

 which appear to be of very late Tertiary age (Pliocene or 

 Pleistocene). Capping the whole are raised coral reefs. Here, 

 •therefore, is part of a continental (or shallow sea) area which has 

 sunk into oceanic depths during the Tertiary period, has received 

 a burden of oceanic deposits, and has risen again to be invested 

 •with a formation of essentially shallow water origin. Certainly 

 geologists have no proof of greater geographical changes than 

 this, though Europe affords evidence of quite as great a change, 

 for in the area of the European chalk we have an instance of 

 similar oceanic conditions to those under which the Barbados 

 earths were deposited ; yet this area was continental land before 

 the Cretaceous period, and has again become so since that 

 period. 



The other oceanic areas which have less than 2000 fathoms of 

 water over them are the Arctic Ocean, the southern part of the 

 Indian Ocean, and part of the North Pacific between America 

 -and Kamchatka. It would appear then that we may claim 

 these regions, together with the Caribbean area and a large part 

 oi the Western Pacific, as areas which have been interchangeable 

 with the present continental surfaces.^ 



Mr. Fisher does not discuss the subterranean structure of the 

 shallow ocean areas, but in his letter already quoted he inclines 

 to think that the crust beneath them is similar to the continental 

 ■crust, and this view is borne out by the structure of certain 

 •oceanic islands ; but though the density and general structure of 

 "the crust may be similar to that of the continents, the condition 

 •of the liquid substratum may not be exactly the same, or rather 

 there may be differences in the force and direction of the 

 •convection currents which traverse the substratum. 



In chapter xxiv. Mr. Fisher does briefly consider the condition 

 •of the substratum in the tracts that lie between the continents 

 and the [deep] oceanic regions. Having shown that, if the 

 density of the substratum is less beneath the ocean than beneath 

 the land, the convection currents must rise beneath the oceans 

 and descend beneath the continents, he points out that there 

 must be a certain space between the lines of ascent and descent 

 where the currents will move more or less horizontally. In this 

 horizontal movement he finds a force capable of exerting strong 

 pressure on the continental crust. Now in some parts of the 



' The ridges in the Central and Southern Atlantic do not come ■within th« 

 category of shallow oceans. 



world the space along which these horizontal currents move may 

 be narrow, but in others it is probably broad : thus, on the east 

 side of the Pacific, where the change from ocean depths to moun- 

 tain heights is rapid, this space is doubtless small, but on the 

 west side of the same ocean, as we have seen, there is a broad 

 intervening area of shallow ocean, and beneath this the currents 

 that move westward may continue to be mainly horizontal till 

 they reach Australia. 



The behaviour of convection currents is so little understood 

 that one cannot predicate much about them ; there would prob- 

 ably be a certain play of ascending and descending currents 

 beneath the broad semi-oceanic area as well as horizontal 

 currents, and very slight changes may cause these to vary in 

 volume and to alter their positions ; such a region is therefore 

 likely to b5 in a state of unstable equilibrium, and its upheaval 

 or further subsidence would depend on the balance that is 

 established between the three sets of currents in the liquid 

 substratum beneath it. 



Another question suggests itself — namely, whether the oceans 

 have always been as deep as they are now. According to Mr. 

 Fisher's results, the mass of the sub-oceanic crust is greater than 

 that of the sub-continental crust, but he gives reasons for thinking 

 that its thickness is not greater, and if this is so, then its density 

 must be greater ; and it is from this he deduces the permanency 

 of the oceans, because it is difficult to conceive of the denser crust 

 becoming less dense, which would be necessary before any part 

 of it could be converted into a continent. But though this 

 difficulty certainly exists, it does not preclude the possibility 

 of the sub- oceanic crust having been originally less dense than it 

 is now ; it may have been growing denser, and there may have 

 been a corresponding increase in the size and depth of the oceans 

 at the expense of the continents. His results, in fact, do not 

 involve the permanency of the present continents, or of the pre- 

 sent relative proportions of land and water surfaces. We are at 

 liberty to imagine a time when there was much more land than 

 there is at present, and when all the oceans were comparatively 

 shallow ; there being at this early period less difference in the 

 comparative density of the sub-oceanic and sub-continental crust. 



We may, in fact, postulate a secular increase in the size of the 

 oceans and in the depth of the ocean basins corresponding to a 

 secular increase in the density of the sub-oceanic crust ; and 

 possibly as a consequence a general increased stability of the 

 whole crust. 



The supposition of a secular increase in the depth of the 

 oceans is in accordance with the evidence of geological history, 

 for if there had been such an increase we should expect to find 

 that oceanic deposits of the modern type were essentially Neozoic 

 formations, and would not occur among Palaeozoic rocks ; and 

 such appears to be the case. At present we do not know of the 

 existence of any purely oceanic limestone that is older than the 

 Cretaceous period ; and among the Palaeozoic rocks there are none 

 which appear to have been formed at any great distance from 

 continental land. 



I think it has now been shown that Mr. Fisher's conclusions 

 do not give unqualified support to the theory of the permanence of 

 oceans, but that, on the contrary, they are consistent with two 

 important limitations of the theory— limitations which had 

 already been suggested by geologists before the publication of 

 Mr. Fisher's book. Thus, Prof. Prestwich has expressed the 

 opinion^ "that it is only the deeper parts of the great ocean- 

 troughs that can claim the high antiquity which is now advocated 

 for them by many eminent American and English geologists" ; 

 and I have suggested the probability that "the tendency of all 

 recent geographical changes has been to deepen the ocean- 

 basins, and to raise the mountain-peaks to higher and higher 

 elevations."^ 



It is therefore satisfactory to find that the results of purely 

 physical and mathematical reasoning, on the one hand, and of a 

 consideration of the geological evidence, on the other hand, are 

 so closely in accord. The importance of this agreement consists 

 in the way it opens for the reconciliation of two opposing 

 geological schools: an important limitation is imposed on the 

 Lyellian belief in the past interchange of oceanic and continental 

 areas ; while the extreme view, held by Dana and others, that 

 there has been no such interchange at all, may be equally far 

 from the truth ; the probability being that truth lies midway 

 between the two extremes. 



It is also worthy of note that the hypothesis of a secular 

 increase in the depth of the oceans and the heights of the moun- 



' " Geology," vol. ii. p. 547. 



2 " The Building of the British Isles," p. 334. 



