Theories of the EarWs Physical Evolution. 371 



depressions whicli occasion the irregular profiles of the earth's 

 surface, have not been relative movements due to a variable amount 

 of convergence towards the earth's centre, but have been absolute — 

 now upwards and now downwards. Plateaux and continents are true 

 uplifts, and ocean bottoms are true subsidences. Now if we regard 

 any uplifted area at the surface as being the base of a cone having its 

 vertex at the earth's centre, such an uplift [if absolute and not relative] 

 would involve an increase in the volume of the cone. But volume is 

 the quotient of mass by density. Therefore an increase of volume must 

 be attended, either by an increase in the quantity of matter, or by a 

 decrease of the density of the cone, or by both together. The 

 problem thus becomes narrowed to the inquiry whether by any 

 processes a rising area can receive an accession of mass, and a sinking 

 area lose mass : or else respectively lose and increase in density. 



Although deposition and denudation may add or subtract matter 

 from an area, these changes will not account for the facts to be ex- 

 plained. The addition or subtraction of matter must be in the sub- 

 terranean regions if it exist at all. We may here have recourse to 

 the suggestion of Babbage and Sir J. Herschel, that the sea-bottom, 

 becoming overloaded by material denuded off the neighbouring 

 land, the plastic matter supposed to exist beneath is protruded in the 

 direction of least resistance, that is towards the unloaded land. 



Thus a static equilibrium is maintained, and the problem becomes 

 a hydrostatic one. For it is quite clear, that if the crust of the 

 earth rests everywhere upon a liquid, or even considerably plastic 

 support, the altitude of every portion of it is determined solely by 

 the laws of hydrostatic equilibrium. But the utmost that the move- 

 ments arising from this cause, as at present alleged, could effect 

 would be the maintaining of the respective areas at their old levels, 

 in spite of denudation on the one hand and the deposition and building 

 up of strata on the other. It does suggest a mode by which a land 

 area may receive beneath it an accession of mass, but only to a very 

 limited extent ; and how an oceanic area may lose mass, but only to 

 an amount equivalent to the gain in stratification. 



There is another way in which the required additions of mass 

 may be conceived to occur, and that is by the permeation of water. 



There is a general tacit assumption, that in the progress of the 

 evolution water gradually penetrates into the profounder depths of 

 the earth, and into the presence of the primal heat. In this manner 

 have been explained the large quantities of water brought up by 

 lava, and incorporated with it, and the veins filled by minerals 

 precipitated from watery solution. This penetration is commonly 

 believed to be effected, not solely by the way of fissures and cracks, 

 but to include the complete permeation of the rocks by water : else 

 how could lavas be supersaturated with it, metamorphism effected by 

 it, and mineral veins filled by segregation from the rock walls or 

 from below? But it is usually supposed with apparent justice, that 

 the penetration is limited as to depth by the repulsive power of 

 heat, which increases at a rate sufficiently rapid to overcome at a 

 greater or less depth the effect of pressure to keep it [the water] in 



