August i8, 1892] 



NATURE 



375 



objects of study of the stratigraphical geologist. If, therefore, 

 the physiognomy of the face of our globe is any real index of 

 the character of the personality of the earth-crust beneath it, 

 these collective geographical features should be precisely those 

 which answer to the collective structural characters of the 

 geological formations. 



In the earlier days of geology one of the first points recog- 

 nized by our stratigraphists was the fact that the formations 

 were successive lithological sheets, whose truncated outcropping 

 edges formed the present surface of the land, and that these 

 sheets lay inclined at an angle one over the other, or as William 

 Smith quaintly expressed it, like a tilted "pile of slices of 

 bread and butter." But as discovery progressed the explana- 

 tion of this arrangement soon became evident. The formations 

 revealed themselves as a series of what had originally been de- 

 posited as horizontal sheets, lying in regular order one over the 

 other, but which had been subsequently bent up into alter- 

 nating arches and troughs {i.e. the anticlines and synclines of 

 the geologist). Their visible parts, which now constitute 

 the surface of our habitable lands, were simply those parts 

 of the formations which are cut by the irregular plane of 

 the present earth's surface. All those parts of the great arches 

 and troughs formerly occurring above that plane have been 

 removed by denudation ; all those parts below that plane lie 

 buried still, out of sight within the solid earth-crust. 



Although in every geological section of sufficient extent it was 

 seen that the anticline or arch never occurred without the syn- 

 cline or trough — in other words, that there was never a rise 

 without a corresponding fall of the stratum, yet it is only of 

 late years that the stratigraphical geologist has come clearly to 

 recognise the fact that the anticline and syncline must be con- 

 sidered together, and must be united as a single crust-wave. For 

 the arch is never present without its complementary trough, and 

 the two together constitute the tectonic, siructtcral, or orographic 

 tiuit, namely, The Fold, the study of which, so brilliantly inaugu- 

 rated by Heim in his " Mechanismus der Gebirgsbildung," is 

 destined, I believe, in time, to give us the clue to the laws which 

 rule in the local elevation and depression of the earth-crust, and 

 furnish us with the means of discovery of the occult causes 

 which lie at the source of those superficial irregularities which 

 give to the face of our globe its variety, its beauty, and its 

 habitability. 



We have said already that this wave or fold of the geologist 

 resembles that of the wave of the physicist. Now we may 

 regard such a wave as formed of two parts, the arch-like part 

 above and the trough-like part below. The length of the wave 

 is naturally the length of the axial line joining the outer 

 extremities of the arch and trough, and passing through the 

 centre, node, or point of origin of the wave itself, which bisects 

 the line of contrary curvature. The amplitude of the wave is 

 the height of the arch added to the depth of the trough. The 

 arch part of such a wave, if perfectly symmetrical, may clearly 

 be regarded as belonging either to a wave travelling to the right, 

 in which case the complementary trough is the one in that 

 direction, or it may be regarded as belonging to a wave travelling 

 to the left, in which case its trough must be the one in that 

 direction. But as in the case of the shore wave, the advancing 

 slope of the wave is always the steeper, and the real centre of the 

 wave must lie half-way down this steeper slope ; so there is no 

 difficulty in recognizing the centre of a geological fold and 

 fixing the real direction of movement. 



The fold of the geologist differs from the ordinary wave of the 

 physicist, essentially in the fact that even in its most elementary 

 conception, as that of a plate bent by a pressure applied from 

 opposite sides, it necessarily includes the element of thickness. 

 And this being the case, the rock sheet which is being folded 

 and curved has different layers of its thickness affected differ- 

 ently. In the arch of the fold the upper layers of the rock sheet 

 are extended, while its lower layers are compressed. On the 

 contrary in the trough of the fold the upper layers are com- 

 pressed and the lower layers are extended. But in arch and 

 trough alike there exists a central layer, which, beyond taking 

 up the common wave-like form, remains practically unaffected. 



But the geological fold has in addition to length and thick- 

 ness, the further element of breadth, and this fact greatly com- 

 plicates the phenomena. 



Many of the movements which take place in a rock sheet 

 which is being folded, or in other words those produced by the 

 bending of a compound sheet composed of many leaves, can be 

 fairly well studied in a very simple experiment. Take an 



NO. I 190, VOL. 46] 



ordinary large note-book, say an inch in thickness, with flexible 

 covers. Rule carefully a series of parallel lines across the edges of 

 the leaves at the top of the book, about \ of an inch apart, and 

 exactly at right angles to the plane of the cover. Then, holding 

 the front edges loosely, press the book slowly from back and 

 front into an S-like form until it can be pressed no further. As 

 the wave grows, it will be noticed that the cross lines which 

 have been drawn on the upper edge of the book remain fairly 

 parallel throughout the whole of the folding process, except in 

 the central third of the book, where they arrange themselves 

 into a beautiful sheaf-like form, showing how much the leaves 

 of the book have sheared or slidden over each other in this 

 central portion. It will also be seen when the S is complete 

 that the book has been forced into a third of its former breadth.- 

 It is clear that the wave which the book now forms must be 

 regarded as made up of three sections : viz. a section forming 

 the outside of the trough on the one side, and a section forming 

 the outside of the arch on the other, and a central or common 

 section, which may be regarded either as uniting or dividing 

 the other two. 



As this experiment gives us a fair representation of what takes 

 place in a geological fold, we see at a glance that the geologist 

 is forced to divide his fold into three parts — an arch limb, a 

 trough limb, and a middle limb— which last we may call the 

 coptila or the septum, according as we regard it as connecting or 

 dividing the other two. Our note-book experiment shows us 

 also that in the trough limb and the arch limb the leaves or 

 layers undergo scarcely any change of relative position beyond 

 taking on the growing curvature of the wave. But the layers in 

 the central part, or septum, undergo sliding and shearing. It 

 will be found also, by gripping the unbound parts of the book 

 firmly and practising the folding in different ways, that this 

 septum is also a region of warping and twisting. This simple 

 experiment should be practised again and again until all these 

 points are apparent, and the various stages of the folding process 

 become clear ; the surface of the book being forced first into a 

 gentle arch-like rise with a corresponding trough-like fall ; then 

 stage by stage the arch should be pushed over on to the trough 

 until the surlaces of the two are in contact and the book can be 

 folded no further. 



In the structure of our modern mountain ranges we discover 

 the most beautiful illustrations of the bending and folding of the 

 rocky formations of the earth-crust. The early results of Rogers 

 among the AUeghanies, of Lory and Favie in the Western Alps,, 

 have been greatly extended of late years by the discoveries of 

 Heim and Baltzer in the Central Alps, of Bertrand in Provence, 

 of Margerie in Lr\nguedoc, of Button and his colleagues in the 

 western ranges of America, and of Peach and Home and others 

 in the older rocks of Britain. The light these researches throw 

 upon the phenomena of mountain structure will be found admir- 

 ably summarized and discussed in the works of Leconte, of 

 Dana, of Daubree, of Reade, of Heim, and finally in the 

 magnificent work of Suess, the "Antlitz der Erde," of which 

 only the first two volumes have yet appeared. 



Looking first at the mountain fold in its simplest form as that 

 ofa bent rock- plate, composed of many layers which have been 

 forced into two similar arc-like forms, the convexities of which are 

 turned, the one upwards and the other down wards, we find in the 

 present mountain ranges of the globe every kind represented. 

 We commence with one in which the arch is represented merely 

 by a gentle swell of the rock sheet, and the trough by an answer- 

 ing shallow depression, the two shading into each other in an 

 area of contrary flexure. From this type we pass insensibly to 

 others in which we see that the sides of the common limb or 

 septum are practically perpendicular. From these we pass to 

 folds in which the twisted common limb or septum overhangs the 

 vertical, and so on to that final extreme, where the arch limb 

 has been pushed completely over on to the trough limb, and all 

 three members, as in our note-book experiment, are practically 

 welded into one conformable solid mass. 



Although the movements of these mountain folds are slow and 

 insensible, and only effected in the course of ages, so that little 

 or no evidence of the actual movement of any single one of them 

 has been detected since they were first studied, yet it is perfectly, 

 plain that when we regard them collectively, we have here crust 

 folds in every stage of their existence. Each example in itself* 

 represents some one single stage in the lifetime of a single fold. 

 They are simply crust folds of different ages. Some are, as it 

 were, just born ; others are in their earliest youth. Some have 

 attained their majority, some are in the prime of life, and some 



