172 



Prof. J. Prestwich. 



and the mass displaced would be transferred to that part of the 

 adjacent area where the outer crust would yield most readily to 

 deformation and upheaval. If the matter displaced were not confined 

 between resisting surfaces, or if the molten layer were of indefinite 

 thickness or of extreme fluidity, the effect would not be local, but 

 would extend as far as the liquidity of the mass allowed of the 

 transmission of the displaced matter, so that it would tend to become 

 attenuated and lost in the larger volume of which it became part. The 

 result is, in fact, a condition dependent on the measure of viscidity. 



A compression in one part should, therefore, be followed by expan- 

 sion in another, and by a deformation of the crust over conterminous 

 areas. These effects are exhibited in the great continental upheavals 

 and depressions so rife in the times immediately preceding our own, 

 and still in a measure of perceptible action over a large part of the 

 world; as, for example, in the instance of the slow uplifting of the 

 northern portions of the Scandinavian and Greenland peninsulas, and 

 the subsidence of their southern portions, while further south again 

 in Labrador the land has been rising. 



This constitutes the essential difference between the disturbances 

 connected with mountain- and volcano-forming. They both result 

 from the contraction due to secular refrigeration, but the one is a 

 process of excessive lateral compression, and the other of turgid 

 swelling of the crust, for all active volcanoes are on raised sedi- 

 mentary strata. In both cases there is tension, although of a different 

 character. In the latter it is slow, steady, and uniform in its action, and 

 where there are permanent points of comparatively slight resistance, 

 as in volcanic ducts, it then readily finds relief in the expulsion of the 

 lava, which is only prolonged until the equilibrium is restored ; then 

 the eruption ceases and the volcano lapses into a state of rest, only to 

 be broken when again there has been an accumulation of the necessary 

 energy. 



The agency of water is confined to the secondary effects I have 

 described — effects perfectly independent of the forces which produce 

 the extravasation of the lava; and while, with the thinner crust of former 

 times, there would be a more frequent extrusion of the igneous rocks, 

 there is probably, with the thicker crust of the present day and its 

 greater resistance, larger stores of underground water and greater 

 explosive eruptions. 



In connexion with this subject, it is a noticeable fact that volcanoes, 

 although not entirely confined to a definite zone, are far more numerous 

 within 50° lat. N. and S. than beyond those limits, while in the 

 northern hemisphere there are, with one exception, none beyond the 

 Arctic circle ; whereas it is beyond that circle, and where we may 

 suppose the crust of the earth to be thickest, that the great continental 

 movements of uphea^ 1 are now most active and maintained. 



