208 



KNOWLEDGE. 



[September 1, 1897. 



THE EDGE OF A CONTINENT. 



By Geexville A. J. Cole, M.R.I. A., F.G.S., Professor of 

 Geology in the Royal Colleye of IScience for Ireland. 



PEOF. ALBERT DE LAPPARENT, in tbe preface 

 to his charming "Ltcons de Geographie Phy- 

 sique,"" points out how in France, of late years, in 

 struction in physical geography has been entrusted 

 to geologists, and how his own book should be 

 regarded as a sort of bridge thrown between two sciences. 

 " The study of the surface-features of the present day," 

 he writes, " must never be separated from the consideration 

 of the past that gave them birth." Sir Archibald Geikie 

 has already impressed this on us in regard to the teaching 

 of geography : f "In dealing with topographical features , the 

 teacher ought, for his own sake as well as that of his pupils, 

 to make himself master of the general geological structure 

 of the country on which he is about to give lessons." 

 And there is no doubt that such studies will impress keenly 

 upon him the lack of repose that is characteristic of 

 existing features. 



It is on the edges of the great continents that we may 

 look for evidence of this instability and change — change 

 rapid enough, at any rate, to be measured by a time-unit 

 of a thousand or even five hundred years. Where the 

 land meets the sea, even the earhest human inhabitants 

 of the district must have become accustomed to the 

 destructive wash of waters, or to the silting up of passages 

 and caves. Rock-shelves, on which one might sit and fish 

 with primitive lines into deep water, became slowly under- 

 mined, and eventually perished in a single night of storm. 

 Grooves between rocky walls, into which canoes could be 

 safely run, became useless through the movement of 

 pebbles or sand from some distant portion of the coast ; 

 and, in time, tbe old men of a settlement could tell even 

 stranger tales, of remains of houses dimly seen at the 

 falling of the tide — relics of antique and doubtless fairy 

 races, now vanished, with all their belongings, in the sea. 



On the other hand, well-arranged beds of sea-shells and 

 sand, far above the level of storm-wash and high water, 

 pointed to a former extension of the sea across the land ; 

 and behind these phenomena lay some of the most con- 

 troverted questions of geology. 



Despite the opposition of those who sat in medifeval 

 libraries, and who judged all nature by the span of their 

 own cabined lives, we now know, as freely as the old-time 

 fishermen, that the huge sandbanks are piled grain by 

 ^rain against the coast, and that the broad-mouthed bays 

 are carved by the swirl of the sea-waves. But the shell 

 beds lying above the level of the sea, and other evidence 

 of changes in the general relations of sea and land, still 

 form matter for serious debate, and still demand the closest 

 observation. 



In the present article we may apply what may be seen 

 on special portions of the coastline to the whole margin of 

 sea and land — to the continental edge itself. From this 

 point of view we soon learn that what we call the mean 

 sea-level is not a surface of uniform curvature, or an 

 ellipsoid enclosing a smooth and theoretical earth, but is 

 bent up or down according to the nearness or remoteness 

 of the continental mass ; so that the sea-level is high 

 against the land, and low in the centre of the oceans. 

 Could we remove the attraction of the continental masses, 

 many oceanic islands would thus disappear beneath the 

 wave's. 



Experiments with the pendulum have proved ao much, 



* Paris: Masson & Cie., 1896. 



+ "The Teaching of Geography," p. 166. (MacmillanA Co.,1887.) 



though local features — the density of rock-masses and so 

 forth — exert a disturbing influence. A pendulum of Such 

 a length that it should, for the particular latitude, swing 

 from one end to the other of its course in one second, is 

 found to swing too slowly at sea-level on the edge of a 

 continent and too rapidly at sea-level on an oceanic island. 

 The pendulum is, in fact, farther away from the centre of 

 the earth In the one place than it is at the other, although 

 both may be upon the same latitude. 



Now, the constant waste of high land-surfaces, through 

 the action of rain and frost and rivers, is reducing the 

 mass of land above the sea-level, and is letting the sea 

 slip back towards more uniform conditions. As the sea 

 retreats, it should leave behind it traces of its former ex- 

 tension, and sea-shells should thus be found high and 

 dry some hundreds of feet up against the coast. 



We may suspect, however, that such action is so very 

 gradual that the denuding agents remove the marine 

 deposits as they come above the level of high water during 

 the retreat of the surface of the sea. Anything like well- 

 marked marine terraces, where an eroded surface is 

 accompanied by old sea-caves, could not be formed by so 

 continuous a process of retreat ; we should be forced to 

 imagine the intervention of a long period when the 

 denudation of the highland was at a standstill. 



Moreover, over long distances of the continental edge, 

 the traces of old sea-levels, left behind in this manner, 

 would be at the same elevation, or would rise steadily 

 towards the region where the greatest masses stood above 

 the sea. Anything like warping of the old shore-lines, 

 such as has been observed in the south of Sweden,* must 

 be due to movement of the solid land itself. 



Ice-masses have naturally been invoked to account for 

 some of the raised beaches ; an unusual accumulation of 

 Polar ice would draw the oceanic waters northward, and 

 would raise the sea-level along our shores. But Lord 

 Kelvin f has shown that the enormous thickness of ice at 

 one time demanded is a physical impossibility in an un- 

 enclosed basin, owing to the outward viscous flow of the 

 material, which tends, even at Polar temperatures, to thin 

 and flatten the whole mass. Nor would all the ice 

 postulated by the extremest glacialist account for the 

 uptilting or local curvature of the shell-beds or terraces 

 that are left behind. The Chais Hills of Alaska, to quote 

 one fine example, are composed of strata containing shells 

 still hving in the adjacent ocean ; j yet these beds, four 

 thousand to five thousand feet in thickness, are bent up so 

 as to dip northward at an angle of ten to fifteen degrees 

 over a distance of about nine miles. We have here, along 

 the coastline of the Pacific, a range of hills three thousand 

 feet in height — we should call them mountains in our own 

 country — produced by a comparatively modern uplift along 

 the continental edge. 



Whether we study the American seaboard, down to the 

 volcanic line of Chile and Peru, or the coral coast of the 

 Indies and Australia, or our own storm-swept western 

 promontories, we find tbe same series of phenomena, the 

 same problems to be solved. It seems generally agreed, 

 by this time, that continental margins are imstable, 

 and that they mark lines of movement in the crust or 

 skin of the earth. The North American continent is 

 actually flanked by mountain-chains formed of intensely 

 crumpled strata ; and South America presents towards the 



* See, for instance, Penck : " Morphologie der Erdoberflache," 

 Bd. II., p. 538. 



+ " Popular Lectures and Addresses," Vol. II., p. 346. 



t I. C. Eussell : " Second Expedition to Mount St. Elias in 1891," 

 Thirteenth Annual Keport, United States G-eological Surrey, p. 25. 



