374 



NATURE 



[August i8, 1892 



•seas ; and finally, that others, like our Chalk and Greensand, 

 were possibly the extremely slow deposits of the more oceanic 

 deeps. 



Nevertheless, after looking at the formations collectively, there 

 remains no doubt whatever in the mind of the geologist that 

 their mechanical members are the results of the aqueous degrada- 

 tion of vanished lands, and that their organic members are the 

 accumulated relics of the stony secretions of what once were 

 living beings. Neither is there any possibility of escape from 

 the conclusion that they have all been deposited by water in the 

 superficial hollows of the sea-bottoms and ocean floors of the 

 earth- crust of their time. 



In the life of every individual stratified formation of the 

 mechanical type we can always distinguish three stages : first, 

 the stage of erosion and transportation, in which the rock frag- 

 ments were worn off the rocks of the higher ground and washed 

 down by rain and rivers to the sea ; second, a stage of deposition 

 and consolidation below the surface of the quiet waters ; and 

 third, a final stage in which the completed rock -formation was 

 bent and upheaved, in part at least, into solid land. In the 

 formations of the organic type three corresponding stages are 

 equally discernible : first, the period of mineral secretion by 

 organized beings ; second, the period of deposition and consoli- 

 dation ; and third, the final period of local elevation in mass. 

 ■But one and all, mechanical and organic alike, they bear in their 

 composition, in their arrangement, and in their fossils, abundant 

 and irresistible evidences that they -were the products, and that 

 now they are the memorials of the physical geography of their 

 time. 



Guided by the principles of Hutton and Lyell, geologists 

 have worked out with great care and completeness 

 the effects of those agencies which rule in the first of 

 these three life-stages in the history of a mechanical formation. 

 No present geological processes are better known to the young 

 geologist than those of denudation, erosion, and transportation, 

 so familiar to us in the eloquent works of our President. 

 They form together the subject-matter of that most wonderful 

 fascinating chapter in geology, which from its modest opening 

 among the quiet Norfolk sandhills sweeps upwards and onwards 

 without a break to its magnificent close on the brink of the 

 gorge of the Colorado. But our knowledge of the detailed 

 processes of deposition and consolidation which rule in the 

 second stage is still exceedingly imperfect, although a flood of 

 light has been thrown up in the subject by the brilliant results 

 of the Challenger expedition. And we are compelled to admit 

 that our knowledge of the operations of those agencies which 

 rule in the processes of upheaval and depression is as yet almost 

 nil ; and what little we have already learnt of the effects of 

 those agencies is the prey of hosts of conflicting theories that 

 merely serve to annoy and bewilder the working student of the 

 science. 



But not one of the formative triad of detrition, deposition, 

 and elevation can exist without the others. No detrition is 

 possible without the previous upheaval of the rock-sheet, from 

 which material can be removed ; no deposition is possible with- 

 out the previous depression of the rock -sheet, which forms the 

 basin in which the fragmentary material can be laid down. 



Our knowledge, therefore, of the origin and meaning of any 

 geological formation whatever, can at most be only fragmentary 

 until this third chapter in the life-history of the geological for- 

 mation has been attacked in earnest. 



Now all the rich store of knowledge that we possess respecting 

 the first stage in the life of a geological formation has been 

 derived from a comparison of certain phenomena which the 

 stratigraphical geologist finds in the rock formations of the 

 past, with correspondent phenomena which the physical geo- 

 .grapher discovers on the surface of the earth of the present. 

 And all that we know of the second stage again has been 

 obtained in precisely the same way. Surely analogy and common 

 sense both teach us that all which is likely to be of permanent 

 value to us as regards the final stage of elevation and depression 

 must first be sought for in the same direction. 



Within the last twenty years or so many interesting and vital 

 discoveries have been made in the stratigraphy of the rock 

 formations, which bear largely upon this obscure chapter of 

 elevation and depression. And I propose on this occasion that 

 we try to summarize a few of these new facts, and then, reading 

 them in conjunction with what we actually know of the physical 

 geography of the present day, try to ascertain how such mutual 

 agreement as we can discover may serve to aid the stratigraphical 



NO- I 190, VOL. 46] 



geologist in his interpretation of the true meaning of the geo- 

 logical formations themselves. We may not hope for many 

 years to come to read the whole of this geological chapter, but 

 we may perhaps modestly essay an interpretation of one or two 

 of the opening paragraphs. 



In the physical geography of the present day we find the ex- 

 terior of our terraqueous globe divided between the two ele- 

 ments land and water. We know that the solid geological 

 formations exist everywhere beneath the visible surface of the 

 lands, but of their existence under the present ocean floor we 

 have as yet no absolute certainty. We know both the form of the 

 surface and the composition of the outer layers of the continental 

 parts of the lithosphere ; we only know as yet even in outline the 

 form of the surface of its oceanic portions. The surface of each 

 of our great continental masses of land resembles that of a long 

 and broad arch-like form, of which we see the simplest type in the 

 New World. The surface of the North American arch is sagged 

 downwards in the middle into a central depression which lies 

 between two long marginal plateaux, and these plateaux are 

 finally crowned by the wrinkled crests which form its modern 

 mountain systems. The surface of each of our ocean floors 

 exactly resembles that of a continent turned upside down. 

 Taking the Atlantic as our simplest type, we may say 

 that the surface of an ocean basin resembles that of a 

 mighty trough or syncline, buckled up more or less centrally 

 into a medial ridge, which is bounded by two long and 

 deep marginal hollows, in the cores of which still deeper 

 grooves sink to the profoundest depths. This complementary 

 relationship descends even to the minor features of the two. 

 Where the great continental sag sinks below the ocean level, we 

 have our gulfs and our Mediterraneans, seen in our type conti- 

 nent as the Mexican Gulf and Hudson Bay. Where the central 

 oceanic buckle attains the water-line, we have our oceanic 

 islands, seen in our type ocean as St. Helena and the Azores. 

 Although these apparent crust-waves are neither equal in size 

 nor symmetrical in form, this complementary relationship be- 

 tween them is always discernible. The broad Pacific depres- 

 sion seems to answer to the broad elevation of the Old World — 

 the narrow trough of the Atlantic to the narrow continent of 

 America. 



Every primary wave of the earth's surface is broken up into 

 minor waves, in each of which the ridge and its complementary 

 trough are always recognizable. The compound ridge of the 

 Alps answers to the compound Mediterranean trough ; the con- 

 tinous western mountain chain of the Americas to the continuous 

 hollow of the Eastern Pacific which bounds them ; the sweep 

 of the crest of the Himalaya to the curve of the Indo-Gangetic 

 depression. Even where the surface waves of the lithosphere 

 lie more or less buried beneath the waters of the ocean and the 

 seas, the same rule always obtains. The island chains of the 

 Antilles answer to the several Caribbean abysses, those of the 

 ^gean Archipelago answer to the Levantine deeps. 



Draw a section of the surface of the lithosphere along a great 

 circle in any direction, the rule remains always the same : crest 

 and trough, height and hollow, succeed each other in endless 

 sequence, of every gradation of size, of every degree of com- 

 plexity. Sometimes the ridges are continental, like those of 

 the Americas ; sometimes orographic, like those of the Hima- 

 laya ; sometimes they are local, like those of the English Weald. 

 But so long as we do not descend to minor details we find that 

 every line drawn across the earth's surface at the present day 

 rises and falls like the imaginary line drawn across the surface 

 of the waves of the ocean. No rise of that line occurs without 

 its complementary depression ; the two always go together, and 

 must of necessity be considered together. Each pair constitutes 

 one of those geographical units of form of which every con- 

 tinuous direct line carried over the surface of the lithosphere of 

 our globe is made up. This unit is always made up of an arch- 

 like rise and a trough-like depression, which shade into each 

 other along a middle line of contrary curvature. It resembles 

 the letter S or Hogarth's line of beauty, and is clearly similar 

 in form to the typical wave of the physicist. Here, then, we 

 reach a very simple and natural conclusion, viz. the surface of 

 the earth-crust of the present day resembles that of a series of 

 crust-waves of different lengths and different amplitudes, more 

 or less irregular and complex, it is true, but everywhere alter- 

 nately rising and falling in symmetrical halves like the waves of 

 the sea. 



Now this rolling wave-like earth-surface is formed of the out- 

 cropping edges of the rock formations which are the special 



