6i6 



NA TURE 



[April 26, 1894 



limb," the "trough-limb," and a central limb of contrary 

 motion, which is known as the "middle limb" or "septum." 

 In the different regions studied by the geologist, the mode in 

 which this middle limb or septum yields varies greatly according 

 ^3 the material which is being folded behaves as if it were 

 elastic, flexible, or rigid : the strata in the septum or middle 

 limb being sometimes sheared, sometimes bent, but in the 

 majority of cases becomes twisted and broken, while the parts 

 of the fold move most easily and rapidly in proportion as the 

 septum approaches the perpendicular. 



Illustrating the behaviour of this middle limb or septum by the 

 corresponding behaviour of its representative in tidal waves, 

 wind waves, and waves of the sea, &c., and in flexible and 

 brittle sheets of material, the author pointed out that in all these 

 cases the wave or fold, however much deformed, always con- 

 sisted in essence of the two reciprocal halves of the arch and the 

 trough ; but as it became more compressed, the deformation 

 became more and more concentrated within the middle or septal 

 portion of the fold. 



These results not only constitute the key of the geological 

 position, but give us a clue to several of the more remarkable 

 secondary phenomena of the earth's surface, and at the same 

 time afford us a means of grouping together and reducing to 

 fairly natural order many of its supposed anomalies. 



From this fresh point of view we now regard the undulations 

 of the earth's surface not only as wave-forms, and consequently 

 each made up of two reciprocal and balanced elements, the one 

 positive and the other negative, but we also look upon them as 

 folds of various degrees of development, all undergoing a 

 progressive deformation. 



The recognisable amount of this deformation in any surface 

 fold affords us a rough i)idex of that especial stage in its life- 

 history which the fold has attained ; and such a fold should 

 present the phenomena characteristic of a typical geological 

 fold at that special stage of its development. 



The counterbalance or dissymmetry of the positive and 

 negative parts of the narrower and more continuous earth-folds 

 is well illustrated in the case of the great western marginal ridge 

 of the Americas. The crest of the Rocky Mountain-Andes 

 plateau is the longest, straightest, and most continuous ridge on 

 the face of the globe ; and it is bordered throughout, as it should 

 be; by its natural reciprocal — the Eastern Pacific depression or 

 trough, which is correspondingly long, deep, straight, and 

 continuous ; and the two together constitute a single crust-fold. 



Where, on the other hand, the component crests of the great 

 compound earth-ridges are short, irregular, and confused, the 

 reciprocal compound depressions are correspondingly short and 

 irregular ; as, for example, the compound arch of the Alpine 

 ranges, when compared with its compound reciprocal, the 

 Mediterranean troughs. 



The same balance of parts of the two component halves of 

 every crust- wave is discernible even in those subordinate exam- 

 ples where, as in the cases of the archipelagoes, the entire wave 

 is almost wholly carried under water in the trough of a larger 

 oceanic wave, for the collective island-arch immediately over- 

 looks its reciprocal — a deep groove in the ocean-floor. Again, 

 where, as in the cases of the Alps and the Himalayas, the sub- 

 ordinate wave is lifted on the back of a grander continental 

 arch completely out of water, its necessary reciprocal or depres- 

 sion, which at first glance appears to be presumably absent, is 

 found by the geologist to be tucked in in the form of a buried 

 valley, for miles below the great mountain ridge, which has been 

 forced forward, beyond, and above it. 



The same rule holds good even when the collective dry-land 

 areas are regarded as constituting a single arch. Where the 

 marginal septal zone of this continental arch dies down insen- 

 sibly towards the North Pole, we have the shallow reciprocal 

 basin of the Arctic Ocean ; but upon the opposite edge of the 

 arch, where the septal slope rises up steeply and boldly, as 

 along the outer and higher and shattered rim of the con- 

 tinents facing the Pacific and Indian Oceans, the«grandly ele- 

 vated but broken crest, the continental wave looks out imme- 

 diately, as theoretically it should do, over its negative reciprocal 

 the most greatly depressed and broken parts of the ocean-floor. 

 In the case of the geological fold, the study of its life-history 

 shows that the region of yielding and fracture is of necessity 

 the middle region or septum, and that the folding movement 

 takes place most swiftly and easily as this septal portion increases 

 in steepness. 



These natural phenomena of the fold we also find paralleled 



in the case of the earth-waves, whether major or minor. The 

 septal areas and lines dividing the two component halves of the 

 great earth-surface folds mark out distinctly the areas and lines 

 of maximum present volcanicity and earthquake movement on 

 the face of the globe. In proportion as the septal slopes 

 are well marked, long, steep, and continuous, or vice versa, so 

 the intensity of crust-movement and vulcanicity seem to vary 

 from region to region. 



The septal area of the seaward edge of the great Rocky- 

 Mountain- Andes fold is not only the septum of the longest and 

 most continuous crust-fold of the present day, but it actually 

 constitutes the longest and mo>t continuous line of present vol- 

 canic and earthquake action. The steep outward septal edge of 

 the collective continental mass of the globe, sweeping from 

 Behring Strait to the East Indies, thence to the Cape of Good 

 Hope and Cape Plorn back to Behring Strait, shows from end 

 to end its littoral or submarine volcanoes ; while the .-xlmost in- 

 sensible septal edge of the collective continental arch facing the 

 shallow Arctic depressions, shows not a single volcano along the 

 gentle septal declivity for the whole of its extent. In obedience 

 to the same law, surface land marking the steeper edges of all 

 (or, in other words, their septal slopes) of the great mountain 

 plateaus of the Old World, where they face their reciprocals 

 (the deeper plains in front of them), from the Bay of Bengal 

 through the Himalayas, Hindoo Koosh to the Alps and the 

 Mediterranean shores, constitutes the most active and typical 

 zone of continental earthquakes. This rule of septal yielding 

 and movement not only obtains when the great earth-surface 

 waves are regarded in section, but also when they are figured 

 in plan. 



The great compound trough or basin of the Pacific shows all 

 along its septal edge dividing it from its reciprocal or comple- 

 ment, namely, the higher parts of the earth's surface which 

 bound it, an almost complete ring of active volcanoes ; and 

 when a projection is made of the entire earth's superficies, 

 having the North Pole as its centre, it is found that this long 

 volcanic band of the Pacific practically divides that surface in 

 two. It is the primary septal band of the earth's superficies, 

 ranging twice from pole to pole. 



It was next shown that the minor local surface wrinkles of the 

 earth-crust are not only folds in section, and domes and basins 

 when seen in plan, but that they comport themselves as folds 

 even when regarded laterally or horizontally ; the line marking 

 the axes of their crests creeping or flowing horizontally outwards 

 and forwards towards the reciprocal deeps in front. In this way 

 the festoon islands which margin the Pacific, and also the 

 outwardly carving shores of the continents, find an additional 

 explanation. 



Finally, it was pointed out that if the theory of the funda- 

 mental character and domination of the fold or wave in the 

 forms of the earth's surface be well founded, it must necessarily 

 include the most conspicuous features of the earth's surface- 

 relief regarded as a whole. The existence of this paramount 

 feature was first made known to us by the recent deep-sea 

 researches, which made it evident that the vertical relief of the 

 earth-surface regarded collectively consisted of two members — 

 namely, the so-called Continental Plateau (of which our present 

 lands are merely the unsubmerged portions), and the so-called 

 Abyssal Region, i2,ooD feet and more in depth ; these two 

 contrasted elements being united normally by a rapid tran- 

 sitional slope, which lies buried from sight, at a depth of from 

 1000 to 2000 fathoms below the sea-level. 



This remarkable phenomenon the author now interpreted 

 as perfectly natural, and indeed inevitable upon the theory of 

 the crust-fold. The Continental Plateau is merely the col- 

 lective arch (or dome) of the entire relief of the globe, and the 

 Abyssal Region is the collective trough (or basin), while the 

 intermediate slope is merely the natural septal slope common to| 

 the two. But, of course, if this view is correct, it follows ol; 

 necessity, from the characteristics of a fold (i) that the line 

 marking the position of the axial horizontal plane separating, 

 the great earth-arch from the great earth-trough must bej 

 about midway down this septal slope ; (2) the entire area ol 

 the surface of the dome must be equal to that of the basin 

 (3) the collective volume of the dome must be equal to that ol 

 the basin ; and (4) that wherever the septal slope is fairlj 

 straight it must coincide in direction]with the nodal lines of the 

 earth's surface. 



It was pointed out by the author that it had already beei 

 satisfactorily demonstrated by the results of the calculations anc 



NO. 1278, VOL. 49] 



