254: PROGRESS OF SCIENCE IN THE CENTURY. 



vents and lines of weakness or fracture in the earth's 

 crust; the latter that their paroxysmal activity, per- 

 haps even their existence, depends upon the prox- 

 imity of water, so that ^ without water no eruption ' 

 might almost be regarded as an axiom." * 



On the other hand, it seems unsafe to lay too heavy 

 a burden on the expansive force of steam, for though 

 steam is invariably present in volcanic discharges, its 

 amount often appears (as in Hawaii) disproportion- 

 ate to the work done. 



" The most probable view is that volcanoes are closely 

 related to those earth movements which have resulted 

 in the flexing and fracturing of strata. All the greater 

 wrinkles of the earth's surface — its ocean-basins, con- 

 tinental plateaus, and mountains of elevation — owe 

 their origin to the sinking-in of the crust upon the 

 cooling and contracting nucleus. The crust yields to 

 the enormous tangential pressure by cracking across 

 and wrinkling up, in various linear directions, and it is 

 along these lines of fracture and flexure that molten 

 matter and heated vapours and gases are enabled to 

 make their escape to the surface. So far, then, geolo- 

 gists are agreed as to the close relation that obtains be- 

 tween fracturing, folding, and volcanic action. But 

 beyond this agreement ceases.^' f 



Study of Ear'thqualces. — In the first half of the 

 nineteenth century most geologists seem to have ac- 

 cepted the conclusion of Humboldt (1815), that 

 earthquakes were closely associated with volcanic 

 action. 



A long observational period in which data as to 



earthquakes were accumulated by many workers, 



such as Alexis Perrey in Dijon, de Rossi in Italy, 



* Bonney, op. cit., p. 283. 



t Prof. James Geikie. Article, Volcanoes, ChamJ)ers'$ 

 Encyclopa'dia. 



