August i8, 1892] 



NATURE 



Z77 



Alaska is ablaze with volcanoes, while the gently inclined At- 

 lantic septum of the eastern fold from Greenland to Magellan 

 Straits shows none, except on the outer edge of the Antilles, in 

 the very region where the slope of the surface 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 complement 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 submarine septa ought also to be 

 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. 



But we have not yet, however, finished with our simple fold. 

 If we draw a line completely round the globe, crossing the 

 .\tlantic basin at its shallowest, between Cape Verde to 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 grandest order. 

 We have one mighty continental arch stretching from Japan to 

 Chili, broken medially by the sag of the Atlantic trough, and 

 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 ofall 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 depres- 

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

 instantaneous and incontrovertible. It is on the shores of 

 Japan, the region of 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 terrestrial fold returns upon itself. It is an endless fold, 



n endless band, the common possession of two sciences. It is 



)logical in origin, geographical in effect. It is the rvedding- 

 iiig of geology and geography, uniting them at once and for 

 ■ever in indissoluble union. 



Such an endless fold, again, must have an endless septum, 

 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 Antarctic and Indian Oceans, as one imperial 

 earth arch, and regard the unbroken watery expanse of the 

 Pacific as its complementary depression, then the circular coastal 

 band of contrary surface flexure between them should constitute 

 the moving master septum of the 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 earth- 



lake band on which are strung the Festoon Islands of Western 



-ia, the band of Mount St. Elias, the Aleutians, Kamtchatka, 

 iic Kuriles, the band 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 



the most simple and elementary conception of the rock fold 



being made up of arch-limb, trough-limb, and twisting but 

 ^iill continuous septum, we are able to connect, in one unbroken 

 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 anticipated, that the wave-like sur- 



NO. I I 90, VOL. 46] 



face of the earth of the present day reflects in its entirety the 

 wave-like arrangement of the geological formations below. On 

 the land we find that the surface arches and troughs answer 

 precisely to the grander regional anticlines and synclines of the 

 subterranean sedimentary sequence ; and it may, I believe, be 

 regarded as certain that the submarine undulations have a 

 similar or complementary relationship. We find in the new 

 geology, as Hutton found in the old, that geography and geology 

 are one. We find, as we suspected, that the physiognomy of 

 the face of our globe is an unerring index of the solid personality 

 beneath. It bears in its lineaments the characteristic family 

 features and the common traits of its long line of geological 

 ancestors. 



Such, it seems to me, is an imperfect account of the intro- 

 ductory paragraphs of that great chapter in the New Geology 

 now in course of interpretation by geologists of the present day ; 

 and we have translated them exactly in the old way by the aid 

 of the only living geological language, the language of present 

 natural phenomena, and I doubt not that sooner or later the 

 rest of this great chapter will be read by the same simple means, 



I have confined myself to-day to the discussion of the charac- 

 teristics of the simple geological fold as reduced to its most 

 elementary terms of arch, trough, and unbroken septum ; for 

 this being clearly understood, the rest naturally follows. But 

 this twisted plate is really the key which opens the entire 

 treasure house of the New Geology in which lie spread around in 

 bewildering confusion facts, problems, and conclusions enough 

 to keep the young geologist and other scientific men busily at 

 work for many a long year to come. 



Into this treasure-house I often wander myself, in the few. 

 leisure hours that I can steal from a very busy professional life ; 

 and out of it I bring now and again heresies that sometimes 

 amuse and sometimes horrify my geological friends. As you 

 have so patiently listened to what I have already said, perhaps 

 you will permit me in a few final sentences to indicate in brief 

 some of those novelties which I see already more or less clearly, 

 and a few of those less novel points on which it appears to me 

 that more light is wanted. My excuse is twofold — first, to 

 furnish material for work and controversy to the young geolo- 

 gists ; and second, to obtain aid for myself from workers in 

 other walks of science. 



The account of the simple rock-fold I have already given you 

 is of the most elementary kind. It presupposes merely the 

 yielding to tangential pressure from front and back, combined 

 with effectual resistance to sliding. But in the layers of the 

 earth-crust there is always, in addition, a set of tangential 

 pressures theoretically at right angles to this. The simple fold 

 becomes ?i folded fold, and the compound septum twists not only 

 vertically but laterally. On the surface of the globe this double 

 set of longitudinal and transverse waves is everywhere apparent. 

 They account for the detailed disposition of our lands and our 

 waters, for our present coastal forms, for the direction, length, 

 and disposition of our mountain-ranges, our seas, our plains, 

 and lakes. The compound arch becomes a dome, its comple- 

 mentary trough becomes a basin. The elevations and depres- 

 sions, major and minor, are usually twinned, like the twins of 

 the mineralogist, the complementary parts being often inverted, 

 and turned through i8o^ (compare Italy with the Po-Adriatic 

 depression). Every upward swirl and eddy has its answering 

 downward swirl. The whole surface of our globe is thus broken 

 up into fairly continuous and paired masses, divided from each 

 other by moving areas and lines of mountain making and crust 

 movement, so that the surface of the earth of the present day 

 seems to stand midway in its structure and appearance between 

 those of the sun and the moon, its eddies wanting 

 the mobility of those of the one and the symmetry 

 of those of the other. In the geology of the 

 earth-crust, also, the inter-crossing of the two sets of folds, 

 theoretically at right angles to each other, gives rise to effects 

 equally startling. It lies at the origin of the thrust-plane or 

 overfault, where the septal region of contrary motion in the 

 fold becomes reduced to, or is represented by, a plane cf con- 

 trary motion. It allows us to connect together under one set of 

 homologies folds and faults. The downthrow side of the fault 

 answers to the trough, the upthrow side to the arch, of our 

 longitudinal fold ; while the fault-plane itself represents the 

 septal area reduced to zero. The node of the fault, and the 

 alternation and alteration of throw, are due to the effects of the 

 transverse folding. 



