TRANSACTIONS OP SECTION E. 797 



of Arctic and temperate Nortli America and Eurasia are further -withdrawn from 

 the edge of the continental plateau than those of lower latitudes. 



In regions where existing coast-lines approach the margin of the plateau, they 

 are apt to run for long distances in one determinate direction, and, whether the 

 coastal area be liigh or not, to show a gentle sinuosity. Their course is seldom 

 interrupted by hold projecting headlands or peninsulas, or by intruding inlets, 

 while fringing or marginal islands rarely occur. To these appearances the northern 

 regions, as everyone knows, oU'er the strongest contrast. Not only do they trend 

 irregularly, but their continuity is constantly interrupted by promontories and 

 peninsulas, by inlets and fiords, while fringing islands abound. But an elevation 

 of some 400 or 500 fathoms only would revolutionise the geography of those 

 regions, and confer upon the northern coast-lines of the world the regularity which 

 at present characterises those of Western Africa. 



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

 regularity primarily to their approximate coincidence with the steep boundary 

 slopes of the continental plateau, while the irregularities characteristic of the coast- 

 line of North-westeru 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 depressed. I have spoken of the general contrast 

 between hi"-h 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 recognise 

 in the much indented sea-board and the numerous coastal islands 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-ea.'itern Asia, and the multitudes of islands that serve to link that 

 contment 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 considerably from that of the land surface. It is, 

 upon the whole, flat or gently undulating. Here and there it swells gently up- 

 wards into broad elevated banks, some of which have been traced lor 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 farther 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 imdoubtedly 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 whicb 

 geology can" speak with any confidence the coast-lines of the world must have been 

 infinitely more irregular than now. In Palaeozoic times relatively small areas of 



