TRANSACTIONS OF SECTION C. 705 



is the steepest. We see at a glance that the vigour of these two great continental 

 folds, like those of our mountain waves, varies directly as the surface gradient of 

 the septum. 



But the geographical surface of North America, considered as a whole, is in 

 reality that of a double arch with a sag or common trough in the middle. We 

 have seen already that this double arch must be regarded as the natural comple- 

 ment of the equally double Atlantic trough. Here, then, if the path of analogy 

 we have hitherto so triumphantly followed up to this point is still to guide us, 

 the basin of the Atlantic must be, not only in appearance but in actuality, formed 

 of two long minor folds of the same grade as the two that form the framework of 

 America, but with their members arranged in reverse order. If so, their sub- 

 marine septa ought also to he lines of movement and of volcanic action. And 

 this is again the case. The volcanic islands of the Azores and St. Helena lie not 

 exactly on the longitudinal crests of the mid-oceanic Challenger ridge, but upon 

 its bounding flanks. 



We have not yet, however, finished with our simple folds. If we draw a 

 line completely round the globe, crossing the Atlantic basin at its shallowest, 

 between Cape Verde and Cape St. Roque, and continued in the direction of Japan, 

 where the Pacific is at its deepest, as the trace of a great circle, we find that we 

 have before us a crust fold of the very highest and grandest order. We have one 

 mighty continental arch stretching from Japan to Chili, broken medially by the 

 sag of the Atlantic trough; and we see that this great terrestrial arch stands 

 directly opposed to its natural complement, the great trough of the Pacific, which 

 is bent up in the middle by the mightiest of all the submarine buckles of the 

 earth-crust, on which stand the oceanic islands of the central Pacific. 



But if this be true, then the septum of all septa on our present earth-crust 

 must cross our grandest earth fold where the very steepest gradient occurs along 

 this line, and it must constitute the centre-point of the moving earth fold, and of 

 greatest present volcanic activity. And where is this most sudden of all de- 

 pressions ? Taught once more by our geological fold, the answer is instantaneous 

 and incontrovertible. It is on the shores of Japan, the mightiest and most active 

 of all the living and moving volcanic localities on the face of our globe. 



But the course of the line which we indicated as forming our grandest terres- 

 trial fold returns upon itself. It is an endless fold, an endless band, the common 

 possession of two sciences. It is geological in origin, geographical in effect. It 

 is the wedding ring of geology and geography, imiting them at once and for ever 

 in indissoluble union. 



Such an endless fold, again, must have an endless septiun, which in the nature 

 of things must cross it twice. Need I point out to the merest tyro in these wedded 

 sciences that if we unite the Old and New Worlds, and Australia, with their inter- 

 mediate sags of the Atlantic and the Indian Oceans, as one imperial earth-arch, and 

 regard the unbroken watery expanse of the Pacific as its complementary depression, 

 the circular coastal band of contrary surface-flexure which lies between them 

 should constitute the moving master septum of the present earth-crust ? This 

 is the ' Volcanic Girdle of the Pacific,' our ' Terrestrial Ring of Fire.' 



Or, finally, if we rather regard the compact arch of the Old World itself as the 

 natural complement of the broken Indo-Pacific depression, then the most active 

 and continuous septal band of the present day should divide them. Again, our 

 law asserts itself triumphantly. It is the great volcanic and earthquake band on 

 which are strung the Festoon Islands of Western Asia — the band of Mt. St. Elias, 

 the Aleutians, Kamtehatka, and the Kuriles — the hand of Fusijama, Krakatoa, 

 and Sangir. The rate of movement of the earth's surface doubtless everywhere 

 varies directly as the gradient. 



We find, therefore, that even if we restrict our observations to the most simple 

 and elementary conception of the rock fold, as being made up of arch-limb, trough- 

 limh, and twisting but still continuous septum, we are able to connect in one un- 

 broken chain the minutest wrinkle of the finest lamina of a geological formation 

 with the grandest geographical phenomena on the face of our globe. 



We find, precisely as we had anticipated, that the wave-like surface of the earth of 



1892. z E 



