120 



Prof. J. Prestwich. 



gravitation, the vertical work of which is resolved into tangential 

 pressures and motion within the thickness of the shell."* 



The crushing of the earth's solid crust along lines of greatest weak- 

 ness, is in this manner considered by Mr. Mallet to evolve heat suffi- 

 cient to fuse portions of the crust and to cause the extrusion of the fused 

 masses. He says, " The result of the crushing is to produce irregular' 

 masses, on the whole tending to verticality, of pulverised rock, heated 

 more or less highly, that may extend to any depth within the solid 

 crust ; but it is only to such depths as water can percolate or infiltrate- 

 by capillarity that the deepest focus of volcanic activity can be found." 



Geologists find many objections to Mr. Mallet's ingenious hypo- 

 thesis. Amongst the most serious, and it seems to me fatal objections, 

 are — (1) that it fails- to explain the centralisation of the heat, for 

 even if fully developed by sudden, and paroxysmal movements, it 

 would, unless confined to a very narrow line, which is not probable,, 

 be dispersed and dissipated throughout the whole mass affected by 

 the pressure ; (2) that there is an absence of volcanoes in the majority 

 of mountain ranges where the pressure and crushing have been of the 

 most powerful character ; (3) while, on the contrary, in many volcanic 

 areas there is little or no evidence of great lateral pressure^, or of 

 much disturbance of the strata beyond faulting. 



The great mountain ranges of the Alps and Pyrenees, where the 

 strata are tilted, contorted, and enormously crushed, do not contain a 

 single volcano ; the strata are highly metamorphosed, yet show no 

 traces of igneous fusion. In the Andes, the volcanoes are mostly 

 situated on flanking ridges, or on the lower grounds at their base, and 

 rarely on the- high central ridges. Volcanoes, in fact, are as often, 

 or more often, on lines of fault than on anticlinal ridges. But lines of 

 fault, even those of the greatest magnitude, show no fused walls, 

 though the formation of slickenside surfaces must, seeing the great 

 friction, have been attended with considerable heat. 



I however entirely agree with Mr. Mallet in two of his proposi- 

 tions, namely, that volcanic action " is only one phase of a unique 

 force which has always been in action .... since our planet 

 was nebulous ;" and in one sense " that without water we can have no 

 volcano, "f 



§ 2. Objections to Steam as the Primary Cause of Volcanic Eruptions. 



The chief objections to the hypothesis that the vapour of water is 

 the primary agent in volcanic action, are — 



1. The difficulties in the way of accounting for the presence of water 

 in the deep-seated volcanic foci. 



* " Phil. Trans.," vol. 163, p. 167. 

 f Op. cit., p. 216. 



