62 A CHAPTER IN THE PHYSICAL GEOGRAPHY OF THE PAST. 
and bending on themselves in a very abrupt and remarkable 
manner. 
The more this folded structure of the earth’s crust is studied the 
more evident it becomes that it has not been brought about by 
any subterranean forces acting vertically upwards. We can only 
find a reasonable explanation of the complicated, and often 
inverted foldings of mountainous districts, by assuming that the 
force was a /atera/ one, and that it has ridged up the rocks, just 
as a piece of paper or a cloth is puckered when it is laid flat upon 
the table and the fingers pressed upon it with a slight sliding 
movement. 
That nearly all the elevations of old sea bottoms into hills and 
mountains have been produced by lateral thrust all geologists are 
agreed, but on the question as to how that lateral thrust has been 
brought about there are at least two distinct opinions. 
The hypothesis, which has up to recently found most favour 
with geologists, and which taken by itself perhaps explains the 
greatest number of observed facts, is the so-called hypothesis of 
Secular Contraction. 
That the earth was originally a molten mass, which has 
gradually cooled from within outwards, is rendered highly pro- 
bable from certain astronomical considerations; and that its 
interior has still a very high temperature is indicated both by 
volcanic phenomena, and by the fact that the deeper we go down 
the hotter it becomes; the increment of temperature being about 
1° Faht. for every 50 feet of depth. The hot interior or nucleus 
must still be cooling down by conduction of its heat through the 
solidified crust and its dissipation into space, and this cooling 
must also mean contraction.. There is, consequently, a constant 
tendency for the interior nucleus to separate itself from the outer 
and cooler shell, and since it is manifest that the shell cannot 
stand alone, it must tend by the power of gravitation to adapt 
itself to the “diminishing circumference of the contracting 
interior,” and in its efforts to do so, great lateral pressure is 
evoked, which bends, breaks, and ridges the crust along certain 
lines. Thus, on this hypothesis, have been produced the great 
