294 SEE— THE CAUSE OF EARTHQUAKES. [October 19, 



neath time has undermined vast caverns ; whole mountains indeed fall in, 

 and in an instant from the mighty shock tremblings spread themselves far 

 and wide from that centre. And with good cause, since buildings beside a 

 road tremble throughout when shaken by a waggon of not such very great 

 weight; and they rock no less, where any sharp pebble on the road jolts 

 up the iron tires of the wheels on both sides. Sometimes, too, when an 

 enormous mass of soil through age rolls down from the land into great 

 and extensive pools of water, the earth rocks and sways with the undula- 

 tion of the water just as a vessel at times cannot rest, until the liquid 

 within has ceased to sway about in unsteady undulations. . . . 



" The same great quaking likewise arises from this cause, when on a 

 sudden the wind and some enormous force of air gathering either from 

 without or within the earth have flung themselves into the hollows of the 

 earth, and there chafe at first with much uproar among the great caverns 

 and are carried on with a whirling motion, and when their force afterwards 

 stirred and lashed into fury bursts abroad and at the same moment cleaves 

 the deep earth and opens up a great yawning chasm. This fell out in 

 Syrian Sidon and took place at ^gium in the Peloponnese, two towns 

 which an outbreak of wind of this sort and the ensuing earthquake threw 

 down. And many walled places besides fell down by great commotions on 

 land and many towns sank down engulphed in the sea together with their 

 burghers. And if they do not break out, still the impetuous fury of the 

 air and the fierce violence of the wind spread over the numerous passages 

 of the earth like a shivering-fit and thereby cause a trembling" (" De Rerum 

 Natura," Lib. VI, Munro's translation). 



III. The Geographical Distribution of Volcanoes and Their 

 Relation to Earthquake Phenomena. 



§ 10. Four fundamental facts to be explained by a theory of 

 volcanoes. 



A satisfactory theory of the cause of volcanic action must account 

 for the following phenomena : 



1. The distribution of some 400 active volcanoes about the mar- 

 gins of the sea, and the numerous eruptions which take place in the 

 sea or on islands, while none at all occur inland at distances exceed- 

 ing about 100 miles from the ocean or equivalent large bodies of 

 water. 



2. The fact that 999 in 1,000 parts of the vapors emitted by 

 • volcanoes is steam, as if produced by the leakage of the oceans, 



near which the volcanic vents always are situated. 



3. Volcanoes are particular mountains, and all mountains follow 

 the seashore as if formed in some way by the action of the sea upon 

 the adjacent land. 



