702 KEPOET — 1892. 



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 latter we may 

 call the copula or the septum, according as we regard it as connecting or dividing 

 the other two. Our note-book experiment, again, 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 these points are 

 clear, and the various stages of the folding process become apparent, 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 surfaces 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 Alleghanies, of Lory and Favre in the 

 Western Alps, have been greatly extended of late years by the discoveries of Heim 

 and Ealtzer in the Central Alps, of Bertrand in Provence, of Margerie in Languedoc. 

 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 admirably summarised 

 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 of a 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 answerin? shallow depres.sion, 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. 



In many cases, due partly to the action of transverse pressures, the septum 

 becomes reduced to a plane of contrary motion or thrust-plane ; Rnd the arch limb 

 and trough limb slide over each other as two solid masses. But here we have no 

 longer a fold, but a fault. 



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 move- 

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

 in the decrepit stages of old age. Finally, those in which all three members — arch 

 limb, trough limb, and septum — are crushed together into a single mass are dead. 

 Their life of individual movement is over. If the earth pressure increases the 

 material which thej' have packed together may of course form a passive part of a 

 later fold, but they themselves can move no more. 



We see that every mountain fold commences first as a gentle alternate eleva- 



