rALe 
A CHAPTER IN THE PHYSICAL GEOGRAPHY OF THE PAST. 63 
lines of elevation of most of our mountain ranges. They may 
not inaptly be compared with the wrinkles on the skin of a drying 
apple, for the skin of the apple becomes wrinkled in its efforts to 
adapt itself to the shrinking interior of the fruit. 
We should certainly expect that the elevation of ridges on the 
earth’s surface, if these are to be looked upon as the expression 
of secular contraction, would take place along the lines of least 
resistance, where, in fact, the earth’s crust is the thinnest ; it is, 
consequently, a little startling at first to find that the great eleva- 
tions have nearly all taken place where sedimentation has been 
the thickest, and where we might expect the crust of the earth to 
be the strongest. The difficulty, however, disappears on examina- 
tion, and for the explanation we will return once more to the 
Carboniferous strata of the Pennine area. These, as we have 
seen, were laid down in a great trough which gradually bent more 
and more downwards as more sediment accumulated in it. Such 
a great and constantly deepening Gepression in the earth’s sur- 
face is called a geosynclinal, and in such troughs have been 
deposited, sometimes to a thickness of miles, the strata which are 
now elevated in our mountain chains. 
As the great geosynclinal bends more and more downwards, 
the first formed and lowest strata are carried through zone after 
zone of constantly increasing temperature, which at last is 
sufficient to melt, or at any rate to soften, the deepest part of the 
inverted arch. The very keystone of the arch is then gone, and 
it is unable to withstand the great lateral strains due to the secular 
contraction of the earth, and forthwith the elevation of the mass 
begins. 
On the second hypothesis, the lateral pressure which has 
brought about the folds and wrinkles in the earth’s crust is 
attributed to the expansion of the mass of sediment when it is 
carried into the zones of higher temperature, as the geosynclinal, 
or great earth trough, bends more and more downwards under 
the weight of the superincumbent strata. This hypothesis has, 
within the last year or so, come into more prominence, owing 
to the appearance of a most suggestive work, by Mr. Mellard 
