August ii, 1892] 



NATURE 



349 



and depressed areas are of primeval antiquity — that they 

 antedate the very oldest of the sedimentary formations. There 

 is abundant evidence, however, to show that the relatively 

 elevated or continental area has been again and again irregularly 

 submerged under tolerably deep and wide seas. But all 

 historical geology assures us that the continental plateau and the 

 oceanic hollows have never changed places, although from time 

 to time portions of the latter have been ridged up and added to 

 the margins of the former, while ever and anon marginal portions 

 of the plateau have sunk down to very considerable depths. We 

 may thus speak of the great world-ridges as regions of dominant 

 elevation, and of the profound oceanic troughs as areas of more 

 or less persistent depression. From one point of vie^v-, it is true, 

 no part of the earth's surface can be looked upon as a region of 

 dominant elevation. Our globe is a cooling and contracting 

 body, and depression must always be the prevailing movement 

 of the lithosphere. The elevation of the continental plateau is 

 thus only relative. Could we conceive the crust throughout the 

 deeper portions of the oceanic depression to subside to still 

 greater depths, while at the same time the continental plateau 

 remained stationaty, or subsided more slowly, the sea would 

 necessarily retreat from the land, and the latter would then 

 appear to rise. It is improbable, however, that any extensive 

 subsidence of the crust under the ocean could take place without 

 accompanying disturbance of the continental plateau ; and in this 

 case the latter might experience in places not only negative but 

 positive elevation. During the evolution of our continent, crustal 

 movements have again and again disturbed the relative level of 

 land and sea, but since the general re-sult has been to increase 

 the land surface and to contract the area occupied by the sea, 

 it is convenient to speak of the former as the region of dominant 

 elevation, and of the latter as that of prevalent depression. 

 Properly speaking, both are sinking regions, the rate of subsidence 

 within the oceanic trough being in excess of that experienced 

 over the continental plateau. The question of the geographical 

 development of coast-lines is therefore only that of the dry lands 

 themselves. 



The greater land masses are all situated upon, but are nowhere 

 coextensive with, the area of dominant elevation, for very con- 

 siderable portions of the continental plateau are still covered by 

 the sea. Opinions may differ as to which fathoms line we 

 should take as marking approximately the boundary between 

 that region and the oceanic depression ; and it is obvious, 

 indeed, that any line selected must be arbitrary and more or less 

 misleading, for it is quite certain that the true boundary of the 

 continental plateau cannot lie parallel to the surface of the 

 ocean. In some regions it approaches within a few hundreds 

 of fathoms of the sea-level ; in other places it sinks for consider- 

 ably more than looo fathoms below that level. Thus, while a 

 very moderate elevation would in certain latitudes cause the land 

 to extend to the edge of the plateau, an elevation of at least \ 

 10,000 feet would be required in some other places to bring j 

 about a similar result. 



Although it is true that the land surface is nowhere co- 

 extensive with the great plateau, yet the existing coast-lines may j 

 be said to trend in the same general direction as its margins. | 

 So abruptly does the continental plateau rise from the oceanic 

 trough, that a depression of the sea-level, or an elevation of the 

 plateau, for lo,ooo feet, would add only a narrow belt to the 

 Pacific coast between Alaska and Cape Horn, while the gain 

 of land on the Atlantic slope of America between 30"^ N.L. and ! 

 40" S.L. would not be much greater. In the higher latitudes of 

 the Northern Hemisphere, however, very considerable 

 geographical changes would be accomplished by a much less 

 amount of elevation of the plateau. Were the continental 

 plateau to be upheaved for 3000 feet, the major 

 portion of the Arctic Sea would become land. 

 Thus, in general terms, we may say that the coast-lines 

 of Arctic and temperate North America and Eurasia are further 

 withdrawn from the edge of the continental plateau than those 

 of lower latitudes. 



In regions where existing coastlines approach the margin of the 

 plateau, they are apt to run for long distances in one determinate 

 direction, and whether the coastal area be high or not, to show a 

 gentle sinuosity. Their course is seldom interrupted by bold 

 projecting headlands or peninsulas, or by intruding inlets, while 

 fringing or marginal islands rarely occur. To these appeaiances 

 the northern regions, as every one knows, offer the strongest con- 

 trast. Not only do they trend irregularly, but their continuity is 

 constantly interrupted by promontories and peninsulas, by inlets 



and fords, while fringing islands abound. But an elevation of 

 some 400 or 500 fathoms only would revolutionize the geography 

 of those regions, and confer upon the northern coast-lines of the 

 world the regularity which at present characterizes those of 

 Western Africa. 



It is obvious, therefore, that the coast-lines of such lands as 

 Africa owe their regularity primarily to their approximate coin- 

 cidence with the steep boundary slopes of the continental plateau, 

 while the irregularities characteristic of the coast-line of North- 

 western Europe and the corresponding latitudes of North 

 America are determined by the superficial configuration of the 

 same plateau, which in those regions is relatively more de- 

 pressed, I have spoken of the general contrast between high 

 and low northern latitudes, but it is needless to say that in 

 southern regions the coast-lines exhibit similar contrasts. The 

 regular coast-lines of Africa and South America have already 

 been referred to, but we cannot fail to recognize in the much 

 indented sea-board and the numerous coastal island of Southern 

 Chili a complete analogy to the fiord regions of high northern 

 latitudes. Both are areas of comparatively recent depression. 

 Again, the manifold irregularities of the coasts of South-eastern 

 Asia, and the multitudes of islands thai serve to to link that con- 

 tinent to Australia and New Zealand, are all evidence that the 

 surface of the continental plateau in those regions is extensively 

 invaded by the sea. 



A word or two now as to the configuration of the oceanic 

 trough. There can be no doubt that this differs very considera- 

 bly from that of the land surface. It is, upon the whole, flat or 

 gently undulating. Here and there it swells gently upwards 

 into broad elevated banks, some of which have been traced for 

 great distances. In other places narrower ridges and abrupt 

 mountain-like elevations diversify its surface, and project again 

 and again above the level of the sea, to form the numerous islets 

 of Oceania. Once more, the sounding-line has made us 

 acquainted with the notable fact that numerous deep depressions 

 — some long and narrow, others relatively short and broad — stud 

 the floor of the great trough. I shall have occasion to refer 

 again to these remarkable depressions, and need at present only 

 call attention to the fact that they are especially well developed 

 in the region of the Western Pacific, where the floor of the sea, 

 at the base of the bounding slopes of the continental plateau, 

 sinks in places to depths of three and even of five miles below 

 the existing coast-lines. One may further note the fact that the 

 deepest areas of the Atlantic are met with in like manner close 

 to the walls of the plateau — a long ridge, which rises midway 

 between the continents and runs in the same general direction as 

 their coast-lines, serving to divide the trough of the Atlantic 

 into two parallel hollows. 



But, to return to our coast-lines and the question of their 

 development, it is obvious that their general trends have been 

 determined by crustal movements. Their regularity is in direct 

 proportion to the closeness of their approach to the margin of 

 the continental plateau. The more nearly they coincide with 

 the edge of that plateau, the fewer irregularities do they present ; 

 the further they recede from it, the more highly are they indented, 

 ! Various other factors, it is true, have played a more or less 

 important part in their development, but their dominant trends 

 were undoubtedly determined at a very early period in the 

 world's history— their determination necessarily dates back, in 

 short, to the time when the great world-ridges and oceanic 

 ; troughs came into existence. So far as we can read the story 

 told by the rocks, however, it would seem that in the earliest 

 , ages of which geology can speak with any confidence, the coa^t- 

 ; lines of the world must have been infinitely more irregular 

 than now. In Palaeozoic times, relatively small areas of 

 i the continental plateau appeared above the level of the sea. 

 Insular conditions everywhere prevailed. But as ages rolled on^ 

 wider and wider tracts of the ptateau were exposed, and this 

 notwithstanding many oscillations of level. So that one may 

 say there has been upon the whole a general advance from 

 insular to continental conditions. In other words, the sea has 

 continued to retreat from the surface of the continental plateau. 

 To account for this change we must suppose that depression of 

 the crust has been in excess within the oceanic area, and that 

 now and again positive elevation of the continental plateau has 

 taken place, more especially along its margins. That move- 

 ments of elevation, positive or negative, have again and again 

 affected our land areas can be demonstrated, and it seems highly 

 probable, therefore, that similar movements may have been, 

 experienced with the oceanic trough. 



NO, 



>, VOL. 46] 



