796 EEPORT— 1892. 



the oceanic depressions owe tbeir origin to unequal radial contraction of the earth 

 in its secular cooling, help us to understand why the larger features of the globe 

 should te 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 distributed 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 

 has gone, there is reason to believe that these elevated and depressed areas are of 

 primeval antiquity — that they antedate the very oldest of the sedimentary forma- 

 tions. 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. "\Ve 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 view, 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 move- 

 ment 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 stationary, 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 posi- 

 tive elevation. During the evolution of our continents, crustal movements have 

 again and again disturbed the relative level of land and sea, but since the general 

 result 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 massss are all situated upon, but are nowhere co-e.xtensive 

 with, the area of dominant elevation, for very considerable portions of the con- 

 tinental 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 

 jnust 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 approaclies within a few hundreds of fathoms of the sea-level ; 

 in other places it sinks for considerably more than 1,000 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 about a similar result. 



Although it is true that the land sm-face is nowhere co-extensive with the great 

 plateau, yet the existing coast-lines may 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 

 10,000 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. lat. and 40° S. lat. 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 3,000 feet, the major portion of the Arctic 

 Sea would become land. Thug, in general terms, we may say that the coast-lines 



