1906.] 



SEE— THE CAUSE OF EARTHQUAKES. 379 



settled by the observations of Fouque, who showed that true flames 

 may arise when the free gaseous mixtures are ignited by red hot 

 stones from the volcano or by the strokes of lightning which play 

 so actively about the orifice of a volcano in eruption. No doubt the 

 dreadful tongues of fire so often seen to radiate from erupting vol- 

 canoes are to be explained very largely by the ignition of free gases 

 by thunderbolts produced by the condensation of clouds of aqueous 

 vapor. 



§ 48. Straho speaks of the volcanoes as safety valves. 



In his " Principles of Geology," 12 edition, Vol. i, p. 25, Lyell 

 remarks that the gifted Amasean geographer, Strabo, alluding to the 

 tradition that Sicily had been separated by a convulsion from Italy, 

 adds (Lib. vl, p. 396, edit. Almelov. Amst. 1707) that in his time 

 the land near the sea in those parts was rarely shaken by earth- 

 quakes, since open orifices exist whereby fire and burning matter 

 and water escape; but formerly, when the volcanoes of ^tna, the 

 Lapari Islands, Ischia and others were closed up, the imprisoned 

 fire and wind might have produced far more vehement movements. 

 " The doctrine, therefore," continues Lyell, " that volcanoes are 

 safety valves, and that the subterranean convulsions are probably 

 most violent when first the volcanic energy shifts itself to a new 

 quarter, is not modern." 



X. Concluded Theory of Earthquakes. 



§ 49. All important earthquakes due to the action of explosive 

 forces within or just under the earth's crust. 



Major Dutton seems to have had an inkling of the process here 



involved when he wrote the following (" Earthquakes," p. 49-50) : 



" It remains now to refer to the possibility that many quakes whose 

 origin is unknown, or extremely doubtful, may, after all, be volcanic. This 

 must be fully admitted, and indeed, it is in many cases highly probable. 

 Evidences that volcanic action has taken place in the depths of the earth with- 

 out visible, permanent results on the surface abound in ancient rock ex- 

 posures. Formations of great geological age, once deeply buried and 

 brought to daylight by secular denudation, show that lavas have penetrated 

 surrounding rock-masses in many astonishing ways. Sometimes they have 

 intruded between strata, lifting or floating up the overlying beds without any 

 indication of escaping to the surface. Sometimes the lava breaks across a 

 series of strata and finds its way into the partings between higher beds. Or 



