rds
the shrinking subcrustal magma, develops immense compressive
stresses in

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its materials, vast geographical areas being involved. When
folding at length takes place along the axis of the elongated
syncline of deposition, the stresses find relief probably for
some hundreds of miles, and the region of folding now becomes
compressed in a transverse direction. As an illustration, the
Laramide range, according to Dawson, represents the reduction of
a surface-belt 50 miles wide to one of 25 miles. The marvellous
translatory movements of crustal folds from south to north
arising in the genesis of the Swiss Alps, which recent research
has brought to light, is another example of these movements of
relief, which continue to take place perhaps for many millions of
years after they are initiated.

The result of this yielding of the crust is a buckling of the
surface which on the whole is directed upwards; but depression
also is an attendant, in many cases at least, on mountain
upheaval. Thus we find that the ocean floor is depressed into a
syncline along the western coast of South America; a trough
always parallel to the ranges of the Andes. The downward
deflection of the crust is of course an outcome of the same
compressive stresses which elevate the mountain.

The fact that the yielding of the crust is always situated where
the sediments have accumulated to the greatest depth, has led to
attempts from time to time of establishing a physical connexion
between the one and the other. The best-known of these theories
is that of Babbage and Herschel. This seeks the connexion in the
rise of the

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geotherms into the sinking mass of sediment and the consequent
increase of temperature of the earth-crust beneath. It will be
understood that as these isogeotherms, or levels at which the
temperature is the same, lie at a uniform distance from the
surface all over the Earth, unless where special variations of
conductivity may disturb them, the introduction of material
pressed downwards from above must result in these materials
partaking of the temperature proper to the depth to which they
are depressed. In other words the geotherms rise into the sinking
sediments, always, however, preserving their former average
distance from the surface. The argument is that as this process
undoubtedly involves the heating up of that portion of the crust
which the sediments have displaced downwards, the result must be
a local enfeeble