472 Notices of Memoirs — Prof. Laptcorth's Address. 



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 compound 

 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 double complement. The so-called 

 unsymmetrical ranges, therefore, which are theoretically constituted merely of arch 

 limb, trough limb, 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 energy and 

 most rapid movement must be that of middle life. In early youth and in old age 

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

 therefore under the most disadvantageous circumstances. But 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 that 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 

 moving 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 thick- 

 ness, 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 in combination 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 composed of sedimentary stuff, it is certainly no 

 longer a stratified rock ; and though it may have been originally purely igneous 

 material, it is certainly no longer volcanic. It is now a manufactured 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 in lesser and lesser dimensions, 

 fold within fold, first down to formations, then to strata, then to laminae, till 

 they disappear at last in microscopic 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 our 

 type 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 antichnes, while the broad 

 sag of the intermediate 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 

 Alleghanies on the other, while the intermediate Mississippi syncline is the coinmon 

 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, the septal 

 areas where most work is now 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 on 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 



