GROWTH OF GEOLOGY. 253 



supporters of Poulett-Scrope, such as Prevost, Hoff- 

 mann, and Montlosier. 



Facts were industriously gathered on both sides, 

 splendid work was done by both schools, but after 

 Lyell's study of the Canary Islands and Madeira in 

 1854, and Poulett-Scrope's papers in 1856 and 1859, 

 von Buch's theory began slowly to give way. Sir 

 Archibald Geikie's work on The Ancient Volcanoes 

 of Great Britain (1897), may be mentioned as a 

 splendid illustration of the achievements of modern 

 volcanology. 



Causes. — The description of active and extinct 

 volcanoes has reached a high degree of perfection ; 

 much has been done in interpreting existing features 

 of the earth in terms of ancient volcanic activity; 

 chemists and petrographers have contributed greatly 

 to our knowledge of volcanic products ; but in regard 

 to the causes of volcanic action there seems still con- 

 siderable uncertainty. 



Standing by itself is the theory of Mallet, that 

 thrusts in the crust (due to cooling of the interior) 

 may have locally crushed rocks to powder, thus de- 

 veloping great heat — sufficient to melt the rock. 

 But proof of the crushing to powder and of subse- 

 quent melting seems absent. '^ This hypothesis, at- 

 tractive as it may be at first sight, appears to be desti- 

 tute of any real foundation." * 



A survey of distribution of volcanoes is of some 

 assistance. " It appears to lead to two inferences — 

 one that volcanoes are commonly arranged in lines; 

 the other, that, when active they are generally in 

 the neighbourhood of large sheets of water. The 

 former fact suggests a connection between volcanic 



* Prof, T. G. Bonney. The Story of our Planet, London, 

 1893, p. 287. 



