British Association, Edinhurgh Meeting — Lajnvorth. 237 
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 tan- 
gential forces act therefore under the most disadvantageous circum- 
stances. 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 favorable 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, 
vary 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 re- 
markably 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 moimtains in themselves. These great arches, scores 
of miles in length, thousands of feet in hight and thickness, must of 
necessity be of enormous weight, capable of crushing to powder the 
hardest rocks over w^hich 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 u)ighty 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 of the original middle limb of the fold ; and whether we call it 
breccia, mylonite, phyllite, or schist, although it maj' be composed of 
sedimentary stuff, it is certainly no longer a straUfled rock ; and though 
it may have been originally purely igneous material, it is certainly no 
\ongev 'ooh((n.ic. 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 fold 
within fold, first down to formations, then to strata, then to lamintv, till 
they disappear at last in microscopic minuteness beyond the Umits 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 actu- 
ally 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 repre- 
sented by the Rockies on the one side and the AUeghauies on the other, 
