348 



NATURE 



[August ii, 1892 



suggestion, but I must not pursue it, for we are dealing with a 

 problem that has as yet baffled the wit of man. 



I have endeavoured to place before you a subject that involves 

 physical and physiological considerations of extreme difficulty. 

 I have endeavoured to show the nature of these difficulties, 

 and although I have not attempted to solve them, I have at all 

 events tried to show reasons why we should refer our diffisrent 

 colour sensations to diffisrences in the incoming nerve impulses 

 rather than to specifically different activities of cells in the 

 visual centre. I have not found it an agreeable task to point out 

 the shortcomings of theories advanced by those for whom I have 

 the deepest regard ; but in the progress of scientific thought it is 

 especially necessary to keep our minds free from the thraldom 

 of established theory, for theories are but the leaves of the tree 

 of science ; they bud and expand, and in time they fade and fall, 

 but they enable the tree to breathe and live. If this address 

 has been full of speculation, I trust you will allow that the 

 scientific use of the imagination is a necessary stimulus to thought, 

 by which alone we can break a path through the dense thicket 

 of the unknown. 



SECTION E. 



geography. 



Opening Address by Prof. James Geiicie, LL.D., 



D.C.L., F.R.SS.L. & E., F.G.S., President of the 



Section. 



Amongst the many questions upon which of late years light 

 has been thrown by deep-sea exploration and geological research 

 not the least interesting is that of the geographical development 

 of coast-lines. How is the existing distribution of land and 

 water to be accounted for ? Are the revolutions in the relative 

 position of land and sea, to which the geological record bears 

 witness, due to movements of the earth's crust or of the hydro- 

 sphere ? Why are coast-lines in some regions extremely regular, 

 while elsewhere they are much indented? About 150 years 

 ago the prevalent belief was that ancient sea-margins indicated 

 a formerly higher ocean-level. Such was the view held by 

 Celsius, who, from an examination of the coast-lands of Sweden, 

 attributed the retreat of the sea to a gradual drying up of the 

 latter. But this desiccation hypothesis was not accepted by 

 Playfair, who thought it much more likely that the land had 

 risen. It was not, however, until after Von Buch had visited 

 Sweden (1806-1808), and published the results of his observa- 

 tions, that Playfair's suggestion received much consideration. 

 Von Buch concluded that the apparent retreat of the sea was not 

 due to a general depression of the ocean-level, but to elevation 

 of the land — a conclusion which subsequently obtained the strong 

 support of Lyell. The authority of these celebrated men gained 

 for the elevation theory more or less complete assent, and for 

 many years it has been the orthodox belief of geologists that the 

 ancient sea-margins of Sweden and other lands have resulted 

 from vertical movements of the crust. It has long been ad- 

 mitted, however, that highly flexed and disturbed strata require 

 some other explanation. Obviously such structures are the 

 result of lateral compression and crumpling. Hence geologists 

 have maintained that the mysterious subterranean forces have 

 affected the crust in different ways. Mountain-ranges, they con- 

 ceive, are ridged up by tangential thrusts and compression, 

 while vast continental areas slowly rise and fall, with little or no 

 disturbance of the strata. From this point of view it is the 

 lithosphere that is unstable, all changes in the relative level of 

 land and sea being due to crustal movements. Of late years, 

 however, Trautschold and others have begun to doubt whether 

 this theory is wholly true, and to maintain that the sea-level may 

 have changed without reference to movements of the lithosphere. 

 Thus Hilber has suggested that sinking of the sea-level may be 

 due, in part at least, to absorption, while Schmick believes that 

 the apparent elevation and depression of continental areas are 

 really the results of grand secular movements of the ocean. The 

 sea, according to him, periodically attains a high level in each 

 hemisphere alternately, the waters being at present heaped up 

 in the southern hemisphere. Prof. Suess, again, believing that 

 in equatorial regions the sea is, upon the whole, gaining on the 

 land, while in other latitudes the reverse would appear to be the 

 case, points out that this is in harmony with his view of a 

 periodical flux and reflux of the ocean between the equator and 

 the poles. He thinks that we have no evidence of any vertical 

 elevation affecting wide areas, and that the only movements of 



NO. I 189, VOL, 46] 



elevation that take place are those by which mountains are up- 

 heaved. The broad invasions and transgressions of the con- 

 tinental areas by the sea, which we know have occurred again 

 and again, are attributed by him to secular movements of the 

 hydrosphere itself. 



Apart from all hypothesis and theory, we learn that the surface 

 of the sea is not exactly spheroidal. It reaches a higher level on 

 the borders of the continents than in mid-ocean, and it varies 

 likewise in height at different places on the same coast. The 

 attraction of the Himalaya, for example, suffices to cause a 

 difference of 300 feet between the level of the sea at the delta of 

 the Indus and on the coast of Ceylon. The recognition of such 

 facts has led Penck to suggest that the submergence of the 

 maritime regions of North-west Europe and the opposite coasts 

 of North America, which took place at a recent geological date, 

 and from which the lands in question have only partially 

 recovered, may have been brought about by the attraction 

 exerted by the vast ice-sheets of the Glacial Period. But, as 

 Drygalski, Woodward, and others have shown, the heights at 

 which recent marine deposits occur in the regions referred to are 

 much too great to be accounted for by any possible distortion of 

 the hydrosphere. The late James Croll had previously endea- 

 voured to show that the accumulation of ice over northern lands 

 during glacial times would suffice to displace the earth's centre 

 of gravity, and thus cause the sea to rise upon the glaciated 

 tracts. More recently other views have been advanced to ex- 

 plain the apparently causal connection between glaciation and 

 submergence, but these need not be considered here. 



Whatever degree of importance may attach to the various- 

 hypotheses of secular movements of the sea, it is obvious that 

 the general trends of the world's coast-lines are determined in 

 the first place by the position of the dominant wrinkles of the 

 lithosphere. Even if we concede that all *• raised beaches," so 

 called, are not necessarily the result of earth-movements, and 

 that the frequent transgressions of the continental areas by 

 oceanic waters in geological times may possibly have been due 

 to independent movements of the sea, still we must admit that 

 the solid crust of the globe has always been subject to distortion. 

 And this being so, we cannot doubt that the general trends of 

 the world's coast-lines must have been modified from time to 

 time by movements of the lithosphere. 



As geographers we are not immediately concerned with the 

 mode of origin of those vast wrinkles, nor need we speculate on 

 the causes which may have determined their direction. It seems, 

 however, to be the general opinion that the configuration of the 

 lithosphere is due simply to the sinking in and crumpling up of 

 the crust on the cooling and contracting nucleus. IBut it must 

 be admitted that neither physicists nor geologists are prepared 

 with a satisfactory hypothesis to account for the prominent 

 trends of the great world -ridges and troughs. According to the 

 late Prof. Alexander Winchell, these trends may have been the 

 result of primitive tidal action. He was of opinion that the 

 transmeridional progress of the tidal swell in early incrustive 

 times on our planet would give the forming crust structural 

 characteristics and aptitudes trending from north to south. The 

 earliest wrinkles to come into existence, therefore, would be 

 meridional or submeridional, and such, certainly, is the prevalent 

 direction of the most conspicuous earth-features. There are 

 many terrestrial trends, however, as Prof. Winchell knew, which 

 do not conform to the requirements of his hypothesis ; but such 

 transmeridional features, he thought, could generally be shown 

 to be of later origin than the others. This is the only specula- 

 tion, so far as I know, which attempts, perhaps not altogether 

 unsuccessfully, to explain the origin of the main trends of 

 terrestrial features. According to other authorities, however, 

 the area of the earth's crust occupied by the ocean is denser than 

 that over which the continental regions are spread. The depressed 

 denser part balances the lighter elevated portion. But why these 

 regions of different densities should be so distributed no one has yet 

 told us. Neither does Le Conte's view, that the continental areas 

 and the oceanic depressions owe their origin to unequal radial 

 contraction of the earth in its secular cooling, help us to under- 

 stand why the larger features of the globe should be disposed as 

 they are. 



Geographers must for the present be content to take the world 

 as they find it. What we do know is that our lands are distri- 

 buted over the surface of a great continental plateau of irregular 

 form, the bounding slopes of which plunge down more or less 

 steeply into a vast oceanic depression. So far as geological 

 research had gone, there is reason to believe that these elevated 



