Notices of Memoirs — Prof. LapwortJi's Address. 471 



. ■ 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 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 conformable mass, are dead. Their 

 life of individual movement is over. If the earth pressure increases, tire material 

 which they have packed together may of course form a passive part of a later fold, 

 but they themselves can move no more. 



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

 becomes reduced to a plane of contrary motion, namely — the over-fault, or thrust- 

 plane, and the arch limb and the trough limb slide past each other as two solid 

 masses. But here we have no longer a fold, but a fault. 



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

 and depression of one or more of the component sheets of the geological forma- 

 tions 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 or 

 septum grows 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 

 gradually over on to the sinking trough part, 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 goes on increasing and the layers of the earth-crust again 

 begin to fold in the same region, 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 crust 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 on one side of the 

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

 groups, like those of the Jura 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 of necessity all slope inwards towards the primary fold. If, there- 

 lore, they originate only on one side of the primary 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 ranges. In this latter case the whole complex range is seen at a glance 



