238 The American Geologist. October, m-i 
while the intermediate Mississippi syncliae is the common property of 
both. Here, then, we reach a much higher grade of fold than the oro- 
graphic 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 where 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 per- 
fectly this agrees with the geological facts goes without saying. It is on 
the steep Pacific side of the western fold that the crushing and crump- 
ling of its rocks are the greatest. It is on the Atlantic side of the eastern 
fold that the contortion and the metamorphism of its rocks are at their 
maximum, 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 bj' 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 earth- 
quakes, while the gently inclined Atlantic septum 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 surface is the 
steepest. We see at a glance that the vigor of these two great continental 
folds, like those of our mountain waves, varies directly as the surface 
gradient of the septum. 
But the geographical surface of North America, considered as a whole, 
is in reality that of a double arch, with a sag or common trough in.the 
middle. We have seen already that this double arch must be regarded 
as the natural complement of the equally double Atlantic trough. Here, 
then, if the path of analogy we have hitherto so triumphantlj^ followed 
up to this point is still to guide us, the basin of the Atlantic must be, not 
only in appearance, but in actuality, formed of two long minor folds of 
the same grade as the two that form the framework of America, but ^vith 
their members arranged in reverse order. If so, their submarine septa , 
ought also to b-) lines of movement and of volcanic action. And this is 
again the case. The volcanic islands of the Azores and St. Helena lie 
not exactly on the longitudinal crests of the mid-oceanic C/«aWf?i;7('r ridge, 
but upon its bounding fianks. 
But we have not yet, however, finished with our simple fold. If we 
draw a line completely round the globe, crossing the Atlantic basin at 
its shallowest, between cape Verde and cape St. Roque, and continue it 
in the direction of .lapan, where the Pacific is at its deepest, as the trace 
