34 OLDHAM: THE STRUCTURE OK THE HIMALAYAS, ETC. 



it might suffice for the original formation of an elevated tract, with 

 the accompaniments of compensation and a condition of isostasy, 

 it would not provide for the maintenance, or re-establishment of 

 the latter after disturbance by the removal of material by denuda- 

 tion from the higher and its deposition on the lower levels. Recently 

 an hypothesis of tumefaction has been proposed by Dr. L. L. 

 Fermor, 1 which appears to be more competent to account for the 

 facts met with in nature; starting with the fact that igneous rocks 

 of the same chemical composition may present themselves in differ- 

 ent forms of mineral constitution, and that these forms vary in 

 specific gravity, he concludes that this variation is the result of 

 the conditions of temperature and pressure under which the different 

 forms of rock consolidated, and distinguishes between the plutonic 

 and the infra-plutonic forms of rock, which may originate from the 

 same magma according to the pressure under which it cooled down 

 to crystallisation. From this concept the consequence follows that 

 in appropriate conditions of temperature and pressure, there might 

 be a passage from one mode of mineral aggregate to another, of 

 different density, accompanied by a corresponding change in volume. 

 As the difference in density of the extreme forms of mineral aggregate 

 may amount to as much as 20 per cent., it seems that we might 

 find in an action of this nature a sufficient explanation for the eleva- 

 tion of even so lofty a range as the Himalayas and, in the opposite 

 direction, for even the greatest depths of the sea, without having 

 to invoke either too great a difference in density, or too large a bulk 

 of material, to fall within limits which are justified by other- 

 observations. Dr. Fermor"s hypothesis would also account for the 

 maintenance of a mountain tract against the action of denudation, 

 for the change in the deeper layers of the crust might easily be a 

 progressive one, and to some extent dependent on the decrease in load. 

 We are not, however, here concerned with a discussion of 

 hypotheses of mountain formation, but with the effect which an 

 hypothesis of origin by tumefaction would have on the question of 

 compensation. This, it will easily be seen, is provided for by the 

 hypothesis, for the protuberance of the surface is the manifesta- 

 tion of a corresponding decrease in density below, and in this way 

 compensation is provided for. It is also obvious that the denuda- 

 tion of the upraised tract and the deposition of the material removed 



l Oeol. Mag. Decade VI, Vol. I, pp. 65-67 (1914) ; cf. also lice. Qeol. Sun: hid,, Vol. 

 XLIII, pp. 41-47 (1913) and XLII, pp. 133-207 (1912). 



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