376 



NATURE 



[August i8, 1892 



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 the 

 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 pres- 

 sures, 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 com- 

 ponent sheets of the geological formations 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. Yield- 

 ing 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 be- 

 comes 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 con- 

 solidate 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 im- 

 mediate 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 

 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, all 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 are of necessary 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 slop- 

 ing side. If they grow on both sides of the original fold, we 

 have the well-known "fan structure" of mountain ranges. 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 centre of the system. But a com- 

 plete 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 limb, and septum, are locally the 

 more natural and the more common. 



NO. I 190, VOL. 46] 



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

 greatest 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 therefore under 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 

 under the most favourable mechanical conditions and with the 

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

 the rate of movement of the septum, like the speed of the storm 

 wind, varies directly as the gradient. 



In our note-book experiment we observed chat little or no 

 change took place in the arch limb and trough limb, while the 

 septum became remarkably sheared and twisted. The same is 

 the case in nature, but here we have to recollect that these mov- 

 ing mountain folds are of enormous size, indeed actual mountains 

 in themselves. These great arches, scores of miles in length, 

 thousands of feet in height and thickness, must of necessity be of 

 enormous weight, capable of crushing to powder the hardest 

 rocks over which they move, while the thrust which drives them 

 forward is practically irresistible. It is plain, therefore, that 

 while the great arch limb and the trough limb of one of these 

 mighty folds move over and under each other from opposite 

 directions, they form together an enormous machine, composed of 

 two mighty rollers or millstones, which mangle, roll, tear, squeeze, 

 and twist the rocky material of the middle limb or septum, 

 which lies jammed in between them, into a laminated mass. 

 This deformed material, which is the characteristic product of 

 the mountain-making forces, is, of course, made up of the stuff 

 or the original middle limb of the fold ; and whether we call it 

 breccia, mylonite, phyllite, or schist, although it may be com- 

 posed of sedimentary stuff, it is certainly no longer a sti-atified 

 rock ; and though it may have been originally purely igneous 

 material, it is certainly no longer volcanic. It is now a manu- 

 factured article made in the great earth mill. 



These mountain folds, however, are merely the types of folds 

 and wrinkles of all dimensions which affect the rock formations 

 of the earth-crust. Within the mountain chains themselves we 

 can follow them fold within fold, first down to formations, then 

 to strata, then to laminae, till they disappear at last in micro- 

 scopic minuteness beyond the limits of ordinary vision. Leaving 

 these, however, for the moment, let us travel rather in the 

 opposite direction, for these mountain folds are by no means the 

 largest known to the stratigraphical geologist. Look at any 

 geological section crossing the continent of North America, and 

 it will be found that the whole of the Rocky Mountain range 

 on its western side and the Alleghany range on the east are 

 really two mighty compound geological anticlines, while the 

 broad sag of the Mississippi Basin is actually a compound 

 geological syncline made up of the whole pile of the geological 

 formations. That is to say, the continent of North America is 

 composed of a pair of geological folds, the two arches of which are 

 represented by the Rockies on the one side and the AUeghanies 

 on the other, while the intermediate Mississippi syncline is the 

 common property of both. Here, then, we reach a much higher 

 grade of fold than the orographic or mountain-making fold, viz. 

 the plateau-making fold or the semi-continental fold, which, 

 because of its enormous breadth, must include a very much 

 thicker portion of the earth-crust than the ordinary orographic 

 fold itself. 



But which must be the real middle limbs of these two American 

 folds, those septal areas where most work is being done and the 

 motion is greatest? 



Taught by what we have already learned of the mountain 

 wave the answer is immediate and certain. They must be on the 

 steeper sides of each of the two folds, namely, those which face 

 the ocean. How perfectly this agrees with the geological facts 

 goes without saying. It is on the steep Pacific side of the 

 western fold that the crushing and crumpling of its rocks is the 

 greatest. It is on the Atlantic side of the eastern fold that the 

 contortion and the metamorphism of its rocks are at their maxi- 

 mum, while in the common and gently sloping trough of both 

 folds, namely, the intermediate Mississippi Valley, the entire 

 geological sequence remains practically unmodified throughout. 



Again, which of these two American folds should be the more 

 active at the present day ? Taught by our study of the mountain 

 wave the answer again is immediate and conclusive. It must 

 be that fold whose septum has the steeper gradient. Geology 

 and geography flash at once into combination. The steeper 

 Pacific septum of the western fold from Cape Horn almost to 



