2/6 HISTORY OF GEOLOGY AND PALEONTOLOGY. 



Wider geographical and geological knowledge has shown 

 the earth's volcanicity to be a phenomenon of universal occur- 

 rence which cannot be explained as a result of occasional local 

 catastrophes. 



Descartes had in 1644 suggested that the friction of 

 inthrown rock-masses might induce processes of fusion, and 

 Franke in 1756 attributed volcanic outbreaks to local 

 shearing in the earth's crust. More recently, conversion of 

 mechanical work into heat was made the basis of a hypothesis 

 by Volger in his book entitled The Earth and Eternity, 

 published in 1857. Volger suggested that both earthquakes 

 and volcanoes were caused by partial collapse and inthrow of 

 rock-material superincumbent upon subterranean cavities. A 

 mechanical theory of a somewhat different character was 

 proposed in 1866 by Mohr. He supposes that certain 

 deep-lying strata in the earth's crust have lost their original 

 consistency either by means of chemical decomposition or 

 from other causes. If these weaker layers be subjected to the 

 pressure of a considerable thickness of overlying rock-deposits, 

 and if, as in the submarine areas, they have to bear in addition 

 the weight of a vast column of water, they may be crushed, 

 heated, and even in some cases melted and ejected at lines of 

 crust-fissures. Mohr referred more particularly the submarine 

 tuffs to this mode of origin. Pfaff wrote in 1871 a paper 

 on "Volcanic Phenomena," in which he opposed Mohr's theory, 

 and said that thermo-dynamic action alone could not generate 

 sufficient heat to fuse rock-masses. 



The English physicist, Robert Mallet, made the most 

 successful attempt to found a mechanical theory of vol- 

 canicity. He assumed that the earth's crust, in consequence 

 of a slow and protracted cooling of the globe, is now of 

 considerable thickness. During the earth's cooling the masses 

 contracted as they solidified, and their contraction created 

 tangential pressures through the crust. According to Mallet's 

 theory, the hotter internal mass of the earth cools and 

 contracts more rapidly than the crust, which is in con- 

 sequence liable to recurring accidents of incrush and inthrow. 

 Tangential pressure is resolved into vertically-acting forces, 

 and folds and corrugates the earth's crust, forming larger and 

 smaller mountain-chains. Fissures develop along the lines of 

 greatest weakness in the crust, and it is chiefly at these that 

 the rocks give way for long distances and are crushed and 



