August ii, 1892] 



NATURE 



355 



far removed from the margin of the continental plateau. The 

 elevation of East Africa for 6000 feet would add only a very 

 narrow belt to the land. This would still leave Madagascar an 

 island, but there are geological reasons for concluding that this 

 island was at a far distant period united to Africa, and it must 

 therefore be considered as forming a portion of the continental 

 plateau. The great depths which now separate it from the 

 mainland are probably due to local subsidence, connected with 

 volcanic action in Madagascar itself and in the Comoro Islands. 

 The southern coasts of Asia, like those of East Africa, approach 

 the edge of the continental plateau, so that an elevation of 

 6000 feet would make little addition to the land area. With 

 the same amount of upheaval, however, the Malay Peninsula, 

 Sumatra, Java, and West Australia, would become united, but 

 without extending much further seawards. Land connection, 

 as we know, existed in Mesozoic times between Asia, Australia, 

 and New Zealand, but the coast-lines of that distant period 

 must have differed considerably from those that would appear 

 were the regions in question to experience now a general eleva- 

 tion. The Archaean and Palaeozoic rocks of the Malay Peninsula, 

 and Sumatra are flanked on the side of the Indian Ocean by great 

 volcanic ridges, and by uplifts of Tertiary strata, which continue 

 along the line of the Nicobar and Andaman Islands into Burma. 

 Thus the coast-lines of that section of the Indian Ocean exhibit 

 a geograi)hical development similar to that of the Pacific sea- 

 board. Elsewhere, as in Hindustan, Arabia, and East Africa, 

 the coast-lines appear to have been determined chiefly by 

 regional elevations of the land or subsidence of the oceanic 

 trough in Mesozoic and Cainozoic times, accompanied by the 

 outwelling of enormous floods of lava. Seeing, then, that the 

 Pacific and Indian Oceans are pre-eminently regions which, 

 down to a recent date, have been subject to great crustal 

 movements and to excessive volcanic action, we may infer 

 that in the development of their coast-lines the sea has played a 

 very subordinate part. The shores, indeed, are largely pro- 

 tected from marine erosion by partially emerged volcanic ridges 

 and by coral islands and reefs, and to a considerable extent also 

 by the sediment which in tropical regions especially is swept 

 down to the coast in great abundance by rains and rivers. 

 Moreover, as the geological structure of these regions assures 

 us, the land would appear seldom to have remained sufficiently 

 long at one level to permit of much destruction by waves and 

 tidal currents. 



In fine, then, we arrive at the general conclusion that the 

 coast-lines of the globe are of very unequal age. Those of the 

 Atlantic were determined as far back as Palaeozoic times by 

 great mountain uplifts along the margin of the continental 

 plateau. Since the close of that period many crustal oscilla- 

 tions have taken place, but no grand mountain ranges have 

 again been ridged up on the Atlantic sea-board. Meanwhile 

 the Palaeozoic mountain-chains, as we have seen, have suffered 

 extensive denudation, have been planed down to the sea- 

 level, and even submerged. Subsequently converted into land, 

 wholly or partially as the case may have been, they now present 

 the appearance of plains and plateaus of erosion, often deeply 

 indented by the sea. No true mountains of elevation are met 

 \\ ith anywhere in the coast-lands of the Atlantic, while vol- 

 canic action has well-nigh ceased. In short, the Atlantic 

 margins have reached a stage of comparative stability. The 

 trough itself, however, is traversed by at least two well- 

 marked banks of upheaval — the great meridional Dolphin 

 Ridge, and the approximately transmeridional Faeroe-Icelandic 

 belt — both of them bearing volcanic islands. 



But while the coast-lands of the Atlantic proper attained 

 relative stability at an early period, those of the Mediterranean 

 and Caribbean depressions have up to recent times been the 

 scenes of great crustal disturbance. Gigantic mountain-chains 

 were uplifted along their margins at so late a period as the 

 Tertiary, and their shores still witness volcanic activity. 



It is upon the margins and within the troughs of the Pacific 

 Ocean, however, that subterranean action is now most remark- 

 ably developed. The coast-lines of that great basin are every- 

 where formed of grand uplifts and volcanic ranges, which, 

 broadly speaking, are comparable in age to those of the 

 Mediterranean and Caribbean depressions. Along the north- 

 east margin of the Indian Ocean the coast-lines resemble those 

 of the Pacific, being of like recent age, and similarly marked by 

 the presence of numerous volcanoes. The northern and western 

 shores, however (as in Hindustan, Arabia, and East Africa), 

 have been determined rather by regional elevation or by sub- 



NO. I I 89, VOL. 46] 



sidence of the ocean-floor than by axial uplifts — the chief crustal 

 disturbaiices dating back to an earlier period than those of the 

 East Indian Archipelago. It is in keeping with this greater 

 age of the western and northern coast-lands of the Indian Ocean 

 that volcanic action is now less strongly manifested in their 

 vicinity. 



I have spoken of the comparative stability of the earth's crust 

 within the Atlantic area as being evidenced by the greater age 

 of its coastal ranges and the declining importance of its volcanic 

 phenomena. This relative stability is further shown by the fact 

 that the Atlantic sea-board is not much disturbed by earth- 

 quakes. This, of course, is what might have been expected, for 

 earthquakes are most characteristic of volcanic regions and of 

 those areas in which mountain-uplifts of recent geological age 

 occur. Hence the coast-lands of the Pacific and the East 

 Indies, the borders of the Caribbean Sea, the volcanic ridges of 

 the Atlantic basin, the lands of the Mediterranean, the Black 

 Sea, and the Aralo-Caspian depressions, the shores of the Red 

 Sea, and vast tracts of Southern Asia, are the chief earthquake 

 regions of the globe. It may be noted, further, that shocks are 

 not only most frequent but most intense in the neighbourhood 

 of the sea. They appear to originate sometimes in the volcanic 

 ridges and coastal ranges, sometimes under the floor of the sea 

 itself. Now earthquakes, volcanoes, and uplifts are all expres- 

 sions of the one great fundamental fact that the earth is a cool- 

 ing and contracting body, and they indicate the lines of weakness 

 along which the enormous pressures and strains induced by the 

 subsidence of the crust upon its nucleus find relief. We cannot 

 tell why the coast-lands of the Atlantic should have attained at 

 so early a period a stage of relative stability — why no a.xial 

 uplifts should have been developed along their margins since 

 Palaeozoic times. It may be that relief has been found in the 

 wrinkling-up of the floor of the oceanic trough, and consequent 

 formation of the Dolphin Ridge and other great submarine fold- 

 ings of the crust. And it is possible that the growth of similar 

 great ridges and wrinkles upon the bed of the Pacific may in 

 like manner relieve the coast-lands of that vast ocean, and 

 prevent the formation of younger uplifts along their borders. 



I have already remarked that two kinds of elevatory move- 

 ments of the crust are recognized by geologists — namely, axial 

 and regional uplifts. Some, however, are beginning to doubt, 

 with Professor Suess, whether any vast regional uplifts are 

 possible. Yet the view that would attribute all such apparent 

 elevations of the land to subsidence of the crust under the great 

 oceanic troughs is not without its difficulties. Former sea- 

 margins of very recent geological age occur in all latitudes, and 

 if we are to explain these by sub-oceanic depression, this will 

 compel us to admit, as Suess has remarked, a general lowering 

 of the sea-level of upwards of 1,000 feet. But it is difficult to 

 believe that the sea-floor could have subsided to such an extent 

 in recent times. Suess thinks it is much more probable that the 

 high-level beaches of tropical regions are not contemporaneous 

 with those of higher latitudes, and that the phenomena are best 

 explained by his hypothesis of a secular movement of the ocean 

 — the water being, as he contends, alternately heaped up at the 

 equator and the poles. The strand- lines in high latitudes, how- 

 ever, are certainly connected with glaciation in some way not 

 yet understood. And if it cannot be confidently affirmed that 

 they indicate regional movements of the land, the evidence, 

 nevertheless, seems to point in that direction. 



In concluding this imperfect outline-sketch of a large subject, 

 I ought perhaps to apologize for having trespassed so much upon 

 the domains of geology. But in doing so I have only followed 

 the example of geologists themselves, whose divagations in 

 territories adjoining their own are naturally not infrequent. 

 From much that I have said, it will be gathered that with regard 

 to the causes of many coastal changes we are still groping in the 

 dark. It seems not unlikely, however, that as light increases 

 we may be compelled to modify the view that all oscillations of 

 the sea-level are due to movements of the lithosphere alone. 

 That is a very heretical suggestion ; but that a great deal can be 

 said for it anyone will admit after a candid perusal of Suess's 

 monumental work, "Das Antlitz der Erde." 



SECTION G. 

 MECHANICAL SCIENCE. 



Opening Address by W. Cawthorne Unwin, F.R.S., 

 M.Inst. C.E., President of the Section. 

 By what process selection is made of a Sectional President of 

 the British Association is to me unknown. I may confess that 



