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exerted when and where the conditions are the most 
favourable. 
Thus the fact that land and sea have repeatedly altered 
their relative positions yields us a problem, which may be 
explained by three causes — 
First. By an oscillation from and to the centre of the earth. 
Secondly. By a gradual but irregular contraction of the 
materials of the earth, particularly at the central 
parts thereof. 
Thirdly. By a gradual expansion of the same materials. 
Either of the latter two suppositions may strike us as 
bordering upon the marvellous, but we shall require some- 
thing very extraordinary to astonish us, if we bear in mind 
such facts as that the Himalaya Mountains, now 26,000 feet 
above the level of the ocean, were within quite a modern 
geological period the bed of a sea. 
We will now examine the probabilities connected with the 
existence of these three supposed causes. Every effect must 
have a cause. If the surface of the earth has been elevated, 
the cause must be the action of some force from below, or 
some attraction from above. If the same surface become 
depressed, the effect must be caused by the withdrawal of 
this force, or by the action of a fresh and more powerful 
agent, and in a contrary direction. It is certainly more 
probable that some force has acted from below, than that an 
attraction from above has caused the elevation of mountains. 
To elevate such a mass as the Alps, the Andes, &c, the force, 
when measured by human ideas, must have been enormous ; 
and this force must have consisted of some material power, 
such as the expansion of the dense masses deeply seated in 
the earth, or of some gaseous or steam-like force which had 
been generated in the interior of the earth. 
It is almost impossible to conceive how any gas could act 
so as to sustain a range of mountains, even if we grant that 
