GROWTH OF GEOLOGY. 255 



and R. and J. W. Mallet in England, was not marked 

 bj any general conclusion of importance. 



In 1873 and 1874, Suess changed the current of 

 opinion by showing that earthquakes recurred in 

 definite lines determined by the structure of the 

 crust, and quite independently of volcanic ac- 

 tion. 



A by-path was opened up by Perrey's theory, 

 suggested by his statistical data, that the attraction 

 of the moon caused what may be called internal 

 tides of the glowing internal fluid mass of the earth^s 

 interior, and that these, rising at times against 

 weaker parts of the heterogeneous unequal crust, 

 caused earthquakes. A somewhat similar tidal 

 theory was elaborated by Rudolf Falb, partly on 

 astronomical grounds, and led him into the dan- 

 gerous field of prophecy. Against both theories it 

 seems sufiicient to urge the enormous probability 

 in favour of the view that the nucleus of the earth 

 is solid. 



The general inclination at present seems to be 

 towards a combination of the conclusions of Hum- 

 boldt and of Suess. On the one hand, earth- 

 quakes may be associated with volcanic activity, — 

 subterranean explosions of gases, the pressure of 

 subterranean flows of lava, the collapse of unsup- 

 ported strata, may set up undulations in the crust. 

 On the other hand, even when volcanoes and earth- 

 quakes occur together in the same country, it has been 

 shown that there may be no demonstrable connec- 

 tion between them. This has been especially well 

 brought out by Prof. J. Milne's seismological work 

 in Japan. He remarks that ^^ earthquakes gener- 

 ally occur in mountainous countries where the moun- 

 tains are geologically young, or in countries where 



