352 TRANSACTIONS OF SECTION C. 
and shrinking core; but Fisher pointed out that the amount of radial shrinking 
could not account even for the present great surface inequalities of the litho- 
sphere, without regard to the enormous lateral shortening indicated by the 
folds in great mountain regions, some of which, like the Himalayan folds, were 
formed at a late date in the Earth’s history, folds which in date and direction 
have no genetic relationship to G. H. Darwin’s primitive wrinkles. Then, 
besides the folding and plication of the crust in some areas, we have to account 
for the undoubted stretching which it has suffered in other places, stretching 
of a kind indicated by faults so common that they are generally known as normal 
faults. It has been estimated by Claypole that the folding of the Appalachian 
range resulted in a horizontal compression of the strata to a belt less than 
65 per cent. of the original breadth. According to Heim the diameter of the 
northern zone of the central Alps is not more than half the original extension 
of the strata when they were laid down in horizontal sheets. De la Beche, in 
his memoir on Devon and Cornwall, which anticipated many problems of more 
than local interest, pointed out that, if the inclined and folded strata were 
flattened out again, they would cover far more ground than that to which they 
are now restricted on the geological map. ‘Thus, according to Dutton, Fisher, 
and others, the mere contraction of the cooling globe is insufficient to account 
for our great rock-folds, especially great folds like those of the Alps and the 
Himalayas, which have been produced in quite late geological times. It is 
possible that this conclusion is in the main true; but in coming to this conclusion 
we must give due value to the number of patches which have been let into the 
old crustal envelope—masses of igneous rock, mineral veins and hydrated pro- 
ducts which have been formed in areas of temporary stretching, and have 
remained as permanent additions to the crust, increasing the size and bagginess 
of the old coat, which, since the discovery of radium, is now regarded as much 
older than was formerly imagined by non-geological members of the scientific 
world. 
The peculiar nature of rock-folds presents also an obstacle no less formidable 
from a qualitative point of view. If the skin were merely collapsing on its shrink- 
ing core we should expect wrinkles in all directions; yet we find great folded . 
areas like the Himalayas stretching continuously for 1,400 miles, with signs of 
a persistently directed overthrust from the north; or we have folded masses 
like the Appalachians of a similar order of magnitude stretching from Maine 
to Georgia, with an unmistakable compression in a north-west to south-east 
direction. The simple hypothesis of a collapsing crust is thus ‘ quantitatively 
insufticient,’ according to Dutton, though this is still doubtful, and it is 
‘qualitatively inapplicable,’ which is highly probable. 
In addition to the facts that rock-folds are maintained over such great 
distances and that later folds are sometimes found to be superimposed on older 
ones, geologists have to account for the conditions which permit of the gradual 
accumulation of enormous thicknesses of strata without corresponding rise of 
the surface of deposition. 
On the other hand, too, in folded regions there are exposures of beds super- 
imposed on one another with a total thickness of many miles more than the 
height of any known mountain, and one is driven again to conclude that uplift 
has proceeded pari passu with the removal of the load through the erosive work 
of atmospheric agents. 
It does not necessarily follow that these two processes are the direct result 
of loading in one case and of relief in the other; for slow subsidence gives rise 
to the conditions that favour deposition and the uplifting of a range results in 
the increased energy of eroding streams. 
Thus there was a natural desire to see if Dutton’s theory agreed with the 
variations of gravity. If the ups and downs are balanced, the apparently large 
mass of a mountain-range ought to be compensated by lightness of material in 
and below it. Dutton was aware of the fact that this was approximately true 
regarding the great continental plateaux and oceanic depressions; but he 
imagined that the balance was delicate enough to show up in a small hill-range 
of 3,000 to 5,000 feet. 
The data required to test this theory, accumulated during the triangulation 
ot the United States, have been made the subject of an elaborate analysis by 
