April 26, 1894] 



NATURE 



61 



been already suggested by one investigator or another. In the 

 speculative parts of the subject, the names of Werner, liatton, 

 De Beaumont, Humboldt, Guyot, Lyell, and Peschel stand 

 conspicuous amongst past investigators ; and, among those of 

 the present day, Le Conte, Dana, Crosby, Dutton, and Gilbert 

 in America ; Heim, Suess, Penck, and Reyer in Germany ; 

 Rcclus, De Lapparent, and Bertrandin France ; and the Geikies, 

 Wallace, Murray, Fisher, Reade, and Mill in Britain. 



Turning first to the general disposition of the recognisable 

 parts of the terraqueous surface of the globe, the author passed 

 in review a few ot the fundamental facts and conclusions worked 

 out by students of the subject, anl showed that it had long since 

 been acknowledged that between all the grander forms of the 

 earth's surface there existed a curious correspondence of shape 

 and of size, combined with a mysterious contrast of geographical 

 arrangements or disposition. Next it was discovered that among 

 the minor elements of surface form, the study of geographical 

 homologies showed us that all the recognisable forms of a higher 

 order discernible up )n the earth's surface are made up of a 

 kind of rhythmic repetition of forms of a lower order possessing 

 in miniature the characteristics of the major forms. These conclu- 

 sions had been practically arrived at by the students of the 

 relief of the globe previous to the recent discoveries of the 

 Challenger and other exploring expeditions, but the result of 

 these deep-sea researches were so strange and so unexpected as 

 almost to dwarf, for the time, these earlier ideas into insigni- 

 ficance. 



Deep-sea researches showed that former ideas of the 

 similarity of size and form between the surfaces of the land and 

 water areas as such must be relinquished. For the mean height 

 of the land was found to be only one-sixth of the mean depth of 

 the ocean; and the entire v)lume of the solid lands above the 

 level of the sea was discovered to be only one-fourteenth of the 

 volume of the ocean waters below that level. Further, what 

 was far more startling and far more important, it was found that 

 the shore-lines of the visible continents by no means mark 

 the true edges of the great ocean-basins ; the dry lands were 

 ascertained to be merely the undrowned portions of one universal 

 continental plateau, the surface of which sinks at first very gently 

 from the shore-line through a shallow water area many miles 

 across, and then plunges rapidly downwards in a sudden slope 

 to the true or abyssal floor of the ocean. This spreads out as 

 a broad undulating plain some twelve thousand feet and more 

 below the sea-level, and descends even still deeper locally in 

 magnificent lake-like hollows to depths of from twenty thousand 

 to thirty thousand feet. These results gave us for the first time 

 a map of the forms of a region of the earth's surface at least 

 twice as large as that of the whole of the dry lands united 

 together. Nothing so important, from a geographical point of 

 view, has been accomplished since the days of Columbus. It is 

 the discovery of a new world. But in this new world it is plain 

 that if the deductions of the earlier students of the earth's sur- 

 face are of any value whatever, they must prove to be equally 

 natural and inevitable. 



Leaving the discussion of these deep-sea discoveries for a 

 while, the author next summarised the fundamental conclusions 

 arrived at by the geologist. The geologist has discovered that 

 in all dry lands of the earth which he has hitherto investigated 

 in detail, the local surface of the country is composed of the 

 outcropping edges of solid rock sheets known as the " geological 

 formations." These formations show distinctly, by their com- 

 position and by the relics of marine life which they contain, 

 that they were originally laid down as layers of gravel, sand, 

 and mud upon the floor of the sea. In other words, the surface 

 of every geological formation must have constituted, at the date 

 when it was deposited, an integral portion of the submarine 

 relief of the earth's surface of its time. But while it is clear 

 that these formations were laid down bel^wthe sea-level and in 

 an approximately horizontal position, they are now found, 

 wherever we can eximine them upon the dry lands, usually far 

 above that sea-level ; and, instead of being horizontal, the sur- 

 face of each formation is now found to be typically warped into 

 great undulations like the surface of a folded cloth, or that of a 

 rolling sea. 



The undulations or wave-like forms of the surfaces of the 

 geological formations are of all degrees of importance, the 

 smaller waves riding on the backs of the larger ones, like 

 ripples on the backs of the sea-waves. Bat in spite of the 

 extreme complexity of this arrangement, it is comparatively 

 iasy of study, for we find the whole to be made up of endless 



NO. I27S, VOL. 49I 



repetitions of one and the same fundamental unit — namely, an 

 undulation or wave-like form ; and the study of the character- 

 istics and life-history of one of these typical undulations gives 

 us, within certain limits, the key to those of all the rest. 



Each simple geological undulation consists of two parts, an 

 arch-like rise and a trough-like fall, and these two reciprocal 

 elements are most naturally and conveniently united by the 

 geologist under the single title of the "crust-wave" or "geo- 

 logical fold." 



This curious wave-like disposition of the surface of any geo- 

 logical formation is apparent, whether we follow that surface, 

 say, from east to west, or whether we follow it from north to 

 south. So that the present surface of a formation is most 

 simply pictured if we regard it as having been bent up into two 

 sets of undulations, the one set crossing the other at right 

 angles. 



With this geological result as a guide, the author returned to 

 the investigation of the main features of the earth's surface. 

 It was pointed out that any straight line drawn completely 

 round the globe from west to east over the earth's surface, 

 either along the equator, the tropics, or along any of the 

 neighbouring parallels, shows a more or less regularly alternat- 

 ing elevation and depression of that surface, of the same 

 general type as the undulations of a geological section. 

 Along these parallels we have three successive elevations, the 

 Americas, Eurafrica, and Asia-Australia ; and three inter- 

 mediate depressions, the ocean-basins of the Atlantic, the Indian 

 Ocean, and the Pacific. That is to say, the broad forms of these 

 surface undulations naturally suggest the theory that the ex- 

 terior parts of the earth-crust are bent into three primary 

 meridional waves ranging practically from pole to pole, each 

 wave consisting of a single rise and a single fall. Again, if 

 the crest of any one of the three meridional continental ridges 

 is followed in a transverse direction, i.e. from north to south, it is 

 found that the surface of the crest itself rises and falls in its turn 

 in three successive undulations. For example, in the case of the 

 ridge of the Americas, it is crossed by the three transverse ridges 

 of North America, South America, anl the Antarctic continent ; 

 and the three transverse depressions of the Arctic Ocean, the 

 Carribbean, and the depression south of Cape Horn. 



Thus (precisely as in the case of the surface of a geological 

 formation) the surface of the earth-crust at the present day is 

 most simply regarded as the surface of a continuous sheet which 

 has been warped up by two sets of undulations crossing each 

 other at right angles. But in the case of the earth-surface, the 

 one set ranges parallel with the equator, and the other ranges 

 from pole to pole. 



By means of a figure giving the natural disposition of the re- 

 sultant forms and nodal lines characteristic of the surface of an 

 elastic film warped by two orthogonal and simultaneous sets of 

 undulations, the author showed how the phenomena apparent 

 upon this cross undulated film suggested at a glance, (i) the 

 forms and disposition of the terrestrial continents ; (2) the 

 triangular shapes of their extremities; (3) the diagonal treads 

 of their shores ; and (4) the courses of the archipelagic lines. 



Carrying the method a stage farther, and breaking up each 

 of the major waves symmetrically in a corresponding manner, 

 the author showed how the subordinate forms so obtained 

 now suggested the typical vertical contour of continents, 

 namely a plain bounded by two marginal ridges and that of 

 an ocean-floor, a submerged plain warped up centrally by a 

 submarine elevation. The same correspondence was shown to 

 hold good even in the broadest grouping of the forms, the col- 

 lective land, and water areas. 



After indicating that we have here the hint that the funda- 

 mental unit of form of the earth's surface is the wave ox fold, 

 and that the surface-contours of the globe are primarily the 

 resultants of the two sets of undulations into which the outer 

 parts of the earth-crust are warped up, the author pointed out 

 that these results up to this point were reliable only as generali- 

 ties, but appear at first sight valueless when we descend to par- 

 ticulars, and he next endeavoured to explain how the known 

 minor variations and anomalies might be perhaps accounted for. 



Returning to the subject of the geological fold, he showed 

 how the various forms of the geological fold, and even many of 

 the phenomena of its life-history, could be imitated by the 

 lateral compression of flexitole sheets of material ; and how as 

 the pressure increases the original symmetrical undulation 

 be:ome3 progressively deformed. The fold divides itself ulti- 

 mately more or less definitely into three parts ; the "arch- 



