Mallet's Theory of Vulcanicity. 43 



result from compression, would scarcely seem admissible, save in 

 a case of absolute homogeneity and equilibrium — if then. It is 

 certainly incompatible with the demonstration made by Professor 

 Belli of Pavia (as quoted by Mallet), to the effect that rigid 

 bodies are weakened by the simultaneous application of ortho- 

 gonal pressures — that no known materials could sustain, under 

 any circumstances, a strain several hundred times greater than 

 that which would crush it if laterally free to yield — that such 

 strains exist in the contracting crust, and that upward deforma- 

 tion must result, if such contraction takes place at all, as the 

 annual loss of heat by the earth compels us to assume is the case. 

 Whether we view the question of rigidity by the light of our 

 direct knowledge of the first twenty-five miles of crust, and of 

 the profound commotions it experiences from time to time, or 

 by that of the demonstrated increase of temperature as we de- 

 scend, rendering it extremely probable that at a comparatively 

 slight depth the rigidity of all materials must be seriously im- 

 paired by a high temperature despite of pressure — or whether 

 we even consider alone the secular loss of heat by radiation, 

 which must result in a contraction affecting unequally the hete- 

 rogeneous couches of which, on any hypothesis, the solid portion 

 of the earth must be composed — it will be difficult to persuade 

 geologists of the actual existence of the " preternatural rigidity " 

 until every reasonable hypothesis that can dispense with this 

 assumption shall have been exhausted. 



Among the objections raised by geologists, the first, and ap- 

 parently gravest, was that of Forbes (Nature, Feb. 6, 1872), 

 who argues the untenableness of Mallet's theory on the ground 

 of the asserted general identity of composition of volcanic 

 ejecta. In fact, from Mallet's point of view, it would seem that 

 lavas might have the composition of any fusible rock whatso- 

 ever in whose strata the crushing might happen to occur, and 

 hence that, if taking place within the sedimentary strata, there 

 ought to be a very great diversity between the ejecta of different 

 vents. 



In his rejoinder Mallet calls attention to the very serious dif- 

 ferences of composition between the extremes of trachytic and 

 basaltic lavas, and to the generally admitted fact that volcanoes 

 are located along axes of upheaval, where the hypogene rocks, 

 and therefore those of the crust proper, approach the sur- 

 face — hence that crushing along these lines of weakness would 

 be by no means likely to produce a greater diversity of lavas 

 than we actually observe. Furthermore, that the " local lake " 

 theory is liable to the same objection, unless the lakes are sup- 

 posed to be located within the (uniform) crust itself. 



He might, it seems to me, have added that the maximum of 



