TEANSACTIONS OF SECTION C. 703 



tioD and depression of one or more of the component sheets of the geological for- 

 mations which make up the earth-crust. This movement is due apparently to the 

 tangential thrusts set up by the creeping together, as it were, of those neighbouring 

 and more resistant parts of the earth-crust which lie in front of and behind the 

 moving wave. Yielding slowly to these lateral thrusts the crest of the fold rises 

 higher and higher, the trough sinks lower and lower, the central common limb 

 •TOWS more and more vertical, and becomes more and more strained, sheared, and 

 twisted. As this middle limb yields, the rising arch part of the fold is forced gra- 

 dually over on to the sinking trough, until at last all three members come into 

 conformable contact, and further folding as such is impossible. Movement ceases ; 

 the fold is dead. We see also from our note-book experiment that the final result 

 of the completion of the fold is clearly to strengthen up and consolidate that part 

 of the crust plate to the local weakness of which it actually owed its origin and 

 position. The fold has by its life-action theoretically trebled the thickness of that 

 part of the earth-plate in which its dead remains now lie. If the lateral pressure 

 "•oes on increasing, and the layers of the earth-crust again begin to fold in the same 

 reo-ion, the inert remains of the first fold can only move as a passive part of a 

 newer fold, either as a part of the new arch-limb, the new trough-limb, or the new 

 septum. As each younger and younger fold formed in this way necessarily 

 includes a more resistant, and therefore a thicker, broader, and deeper sheet of the 

 earth-crust, we have here the phylogenetic evolution of a whole family of crust 

 folds, each successive member of which is of a higher grade than its immediate 

 predecessor. 



But it very rarely happens that the continuous plate in which any fold is 

 imbedded is able to resist the crust-creep until the death of the first fold. Usually, 

 long before the first simple fold is completed, a new and a parallel one rises in 

 front of it normally on the side of the trough limb ; and the two grow, as it 

 were, henceforward, side by side. But the younger fold, being due to a greater 

 pressure than the older, must of necessity be of a higher specific grade, and the 

 two together form a generic fold in common. 



Our present mountain systems are all constituted of several families of folds, 

 formed in this way, of different gradations of size, of different dates of origin, and of 

 different stages of life evolution ; and in each family group the members are related 

 to each other by this natural genetic affinity. 



Sometimes the new folds are formed in successive order onty on one side of the first 

 fold, and then we have our unilateral (or so-called unsymmetrical) mountain groups, 

 like those of the Jiu-a and the Bavarian Alps. Sometimes they are formed on both 

 sides of the original fold, and then we have our bilateral (or so-called symmetrical)' 

 ranges, like the Central Alps. In both cases the septa of the aged or dead folds 

 are of necessity all directed inwards towards the primary fold. If, therefore, they 

 originate only'on one side of the fold, our mountain group looks unsymmetrical, 

 with a very .steep side opposed to a gently sloping side, If they grow on both 

 sides of the original fold, we have the well-known ' fan structure ' of mountain 

 rano'es. In this case the whole complex range is seen at a glance to be a vast 

 compound arch of the upper layers of the earth-crust, keyed up by the material of 

 the dead or dying folds, which, by the necessities of the case, constitute mighty 

 wedges, whose apices are directed inwards towards the centres of the system. But 

 a complete arch of this kind is in reality not a single fold, but a double one, with a 

 septum on both sides of it, and it requires two troughs, one on each side of it, as 

 its natural complement. The so-called unsymmetrical ranges, therefore, which are 

 constituted merely of arch limb, trough hmb, and septum, are locally the more 

 natural and the more common. 



It is clear that in the lifetime of any single fold, its period of greatest effective 

 energy and most rapid movement must be that of middle life. In early youth the 

 lateral pressure is applied at a very small angle, and the tangential forces act there- 

 lore tinder the most disadvantageous circumstances. In the middle life of the fold 

 the arch limb and the trough limb stand at right angles to the septum, and the 

 work of deformation is then accomplished tmder the most favourable mechanical 

 conditions, and with the greatest rapidity. That is to say, the activity of the fold 



