Elevation and Subsidence of the Surface of the Earth. 403 



lessened; but this reduction of pressure will spread beyond the 

 elevated district, and will subvert the equilibrium in all surround- 

 ing areas, which will necessarily subside, and equilibrium will 

 only be restored when the mass of the subsided portion is equal 

 to the mass of the elevated portion. Subsidence must therefore 

 be a necessary consequence of elevation, and vice versa, unless the 

 increase or relief of pressure is general all over the surface of 

 the globe. A general increase of pressure is impossible, as it 

 implies an increase in the total heat of the interior of the earth. 

 A general decrease of pressure is doubtless brought about 

 by the radiation of heat from the earth into space ; but this 

 decrease must have been, at any rate since tertiary times, so 

 small that it is totally inadequate to account for the great 

 changes that have since then taken place*. The late Mr. W. 

 Hopkins and others have also shown that the results that we 

 should expect to follow from a general decrease of the intei-nal 

 pressure do not at all accord with the phenomena we find to 

 exist on the surface of the globe, especially in the general direc- 

 tion of the depressions, in the occurrence of many minor oscilla- 

 tions in the same place, and in the thickest or heaviest deposits 

 having been elevated instead of depressed. Where, then, are we 

 to look for the agencies that are at work to disturb this equili- 

 brium, and so give rise to the movements which we know to 

 have taken place repeatedly over many parts of the earth^s 

 surface ? 



There are two, and only twof, agents constantly at work to 

 produce this effect; these are denudation and deposition. The 

 former, as I shall presently vshow, has comparatively little effect ; 

 for it necessarily brings into play another force, which counter- 

 acts, or more than counteracts, the diminution of pressure caused 

 by the removal of matter ; and it is to deposition that we must 

 look as the principal cause of all the changes that have taken 

 place on the surface of the globe. This, as I have already stated, 

 has been pointed out by Mr. Babbage and Sir J. Herschel ; but 

 it still remains to show its adequacy. We learn from the laws 

 of the conduction of heat that points of equal temperature in the 

 interior of the earth must be arranged in more or less spherical 



* Mr. Lesley has deduced that the earth ought to have shrunk 2 per 

 cent, linearly to account for the observed geological phenomena. Upon 

 this Mr. P. Pierce remarks (see 'Nature/ leb. 16, 1871) that this involves 

 a cooling of not less than 2000° C, which would melt the rocks that are 

 supposed to have shrunk. 



t Terrestrial magnetism may also have some effect ; but it must be very 

 small, as the magnetic intensity in London is only about one 6000th part 

 of that of a single Daniell cell ; and as the directions of this force are not 

 constant, the effects that it is capable of producing will be still further 

 reduced. 



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