September 1, 1897.] 



KNOWLEDGE. 



209 



Pacific its magnificent backbone of the Andes. Tlie 

 Pacific as a whole is ringed about with earth-folds, the 

 details of which indicate a spreading of the continents at 

 the expense of the ocean-basin ; and the progress of 

 movement along these lines of weakness is marked by 

 violent volcanic action. The Atlantic, on the other hand, 

 shows no such symmetry of structure ; * but the whole of 

 the western lands of Europe, from Norway to the Spanish 

 plateaus, may be regarded as an uplifted rim to the 

 huge compound continent on the east. The chains that 

 run east and west, such as the axis of the Pyrenees and 

 the post-Carboniferous folds of Kerry, have been broken 

 oS or bent down towards the depressions of the Atlantic ; 

 and the tendency of earth-movement in late geological 

 times is probably shown by the volcanic line running 

 almost north and south, from the Faroe Islands through 

 the Inner Hebrides, the vents of Antrim, and onward to 

 the Wolf Eock off Cornwall. May we not also include 

 the north and south line of Pliocene volcanoes, the Monts 

 Dume of Central France, as an indication of the system 

 of fractures which determined our present continental edge '? 



Prof. Suessf has insisted much on the contrast between 

 the folded margins of the Pacific and those of the 

 Atlantic, the latter ocean having no relation to the growth 

 of mountain-chains. Along the Pacific, work is evidently 

 being done, and the deep oceanic depressions bordering 

 the continental edges are the submerged limbs of the folds 

 that form those edges. The late Prof. J. D. Dana long 

 ago pointed out how the earth-ridges and the long oceanic 

 grooves were related to one another, the greater mountain- 

 chain having the deeper depression along its flank. In a 

 word, as he urged, the highest mountains face the deepest 

 portions of the ocean, and tlieir steeper flanks descend 

 towards the oceans. 



An aggravating number of exceptions occurs, as Prof. 



Flo. 1. — Fold iu Limestoue, LougliaUiuuv, Co. Diihlin, ilhistrsLting 

 the formation of a Continental Edge by elevation, a corresponding 

 Oceanic Depression being formed by tlie lower limb of tbe fold. 



HeilprinJ has pointed out ; but he agrees with Dana that 

 in many of these cases the mountains have formed the 

 borders of oceanic areas in recent geological times. All 

 round the world each great earth-fold is completed by its 

 sunken limb,§ and the sea is likely to flow into the hollow 



* See Suess, " Antlitz der Erde," Bd. II., pp. 164-6, and 258,261 ; 

 and Lapworth, Address to Geological Section, British Association, 

 Nature, Vol. XLVI (1892), p 377. 



•f" 'See also Penck, op. cit.y p. .570. 



% " The Earth and its Story," p. 18.5, (Bojton, 1896.) 



§ See Lapworth, op. eit., p. 375. 



thus provided, and to move outward or inward on the 

 continental edge according as the fold is thrust forward or 

 retreats. (Fig. 1.) A fold may even become dead; com- 

 pression may have done its worst upon that line of the 

 crust, and the region that once was weak may have become 

 strengthened, as it were, by a cicatrice. The diflfereutial 

 movement in the crust is transferred to new directions, 

 and the whole of the older folds, the mountain-ridges and 

 the ocean grooves, becomes uplifted as a portion of a 

 complex continent. 



Again — as Suess has suggested for the Atlantic — an 

 oceanic area, as we now know it, may be a vast region of 

 depression, bounded by a series of fractures (faults) along its 

 margin. The continental edge is thus formed of material 





Fig. 2. —Coastline in the sonth-west of Scotland, showing 

 typical Fjords and accompanying Islands. The latter are nsually, 

 formed liy the submergence of the ro/s, or the passes from one valley 

 to another, the intervening higher portions of the valley. wall 

 remaining above the sea as islands. (Distance from nortli to south, 

 one liundred and five miles.) 



left behind on the " upthrow " side of the fault, and the 

 fault-plane itself approximately forms its seaward face. This 

 face cuts across earlier features, and is only occasionally in- 

 fluenced by them. Earth-movement has again determined 

 the position of the continental edge upon the map, but no 

 definite line of folding occurs to emphasise the margin. 



On such a coast, fjords will be frequent, owing to the 

 dragging downwards of the margin of the land ; the river 

 valleys are lowered beneath the sea, and the long character- 

 istic inlets result, so well known on the coast of Scotland 

 and of Norway (Fig. 2). The old shorelines that can be 

 traced at varying heights along their flanks point to a 

 time, or to successive times, when the depression was 

 greater than now, or when the sea (according to some 

 writers) was held at a higher level against the continent. 

 The variable height, however, of the terraces in different 

 fjords points to uplift as the cause of their present relation 

 to the sea. The fjord, then, was at one time even deeper 

 than it is at the present day. Whether the continental 

 border is due to folding or to faulting, it is easy to reaUse 

 how its present position represents the sum of many 

 movements, some involving slow depression, others slow 



