68 The Causes and Phenomena of Earthquahes. 



of extinct volcanos ; as well as between Cairo and Suez, and at 

 Aden ; besides the not springs of Tor and the active crater of 

 G-ebel Tir. 



In general the most violent shocks are not found to take place in 

 regions immediately near volcanic foci, and they have been felt 

 on the ocean where the depth is great, and where no visible 

 phenomena appear on the surface. 



The earth wave is a true undulation of the earth's crust, the 

 direction of the shock varying from verticality to horizontality, 

 the motion being direct. 



Sounds of various kinds accompany, precede, or follow some 

 shocks, whilst others are not so attended. No sea wave of earth- 

 quakes has ever been observed in any earthquake where the 

 centre was inland. But a forced wave may occur when an inland 

 shock plunges into the ocean. 



I have chosen these facts from Mr. Mallet, out of many others, 

 all of which he demonstrates by examples, and which are not 

 referred to by Hopkins, in order that, if possible, some ground 

 may be had for the explanation of certain shocks felt in Australia. 



And I now add to these two other facts generally admitted, 

 that earthquake shocks frequently follow the bases of mountain 

 chains, and vary in velocity of transmission according to the 

 nature of the geological formations. Thus, in loose beds of clay, 

 gravel and sand, the motion is slowest ; in solid rocks the motion 

 is quickest ; and in superficial beds over them somewhat less. 

 Known rates have varied from 900 to 6586 feet per second. 

 Eesults, lower — Mr. Mallet says — than the theory would suggest. 



Respecting the effects of elevation of coasts and inland tracts 

 by earthquakes, there is no doubt whatever that such events do 

 take place even in recent times, as they must have occurred in 

 earlier epochs. But caution must be observed in applying this 

 explanation. There is not time now to quote examples, But one 

 source of elevation may be a gradual heating of a solid mass 

 below ; and experiments have been made to show the effect of 

 such expansion by heat, and contraction on coasts. And it is on 

 this principle that the difference of level, at different periods,, of 

 the temple of Jupiter Serapis, has been satisfactorily explained. 

 It is proved that if the temperature of a mass of sandstone a 

 mile iu thickness were raised 200° Fahrenheit, it would expand so 

 as to raise the the rock above it ten feet. And if a mass of the 

 earth's crust of similar description, 50 miles thick, were raised 

 600° or 800° F., then an elevation might take place of 1000 or 

 1500 feet. And conversely, cooling would produce depression. 

 Bischoff* argues that in passing from a molten to a solid state granite 

 would contract ten per cent. He also says, that if carbonic acid 

 gas percolated silicated alkaline rocks, there would be such an 

 expansion as would affect the volume of altered ro#ks, producing 



