704 KEPOET— 1892. 



and the rate of movement of the septum, like the speed of the storm wind, vary 

 directly as the p-radient. 



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 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 in conjunctiou 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 that 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 

 stufi' of the original middle limb of the fold ; and whether we call it breccia, 

 mylonite, phyllite, or schist, although it may be composed of sedimentarj' stuff, 

 it is certainly no longer a stratified rock ; and though it may have been originally 

 purely igneous material, it is certainlj- 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 afl'ect 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 ordinaiy 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 t^-pe continent of North America, and it will be 

 found that the whole of the liocky Mountain range on its western side and the 

 Alleghany ranges on the eastern are really two mighty compound geological anti- 

 clines, while the central sag of the Mississippi Basin is a compound geological 

 synchne 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 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 actual middle limbs of these two American folds, the 

 septal areas where most work is being done and the motion is greatest ? 



Taught by what we have already learnt of the mountain wave, the answer is 

 immediate and certain. They must be the steeper sides of each of the two arches, 

 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 metamorphisra of its rocks are at their 

 maximum ; while in the common and gently sloping trough of both folds, namely, 

 in the intermediate Mississip])i Valley, the entire geological sequence remains prac- 

 tically 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 Alaska, is ablaze with 

 volcanoes, or creeping with earthquakes ; while the gently inclined Atlantic septiuu 

 of the eastern fold, from Greenland to Magellan Straits, shows none, except on the 

 outer edge of the Antilles, in the very region where the slope of the septal surface 



