J". Starkie Gardner — Underground Seat. 401 



What directly concex-ns our present inquiry, however, is whether 

 this heat is at a depth which would make it impossible with our 

 present appliances to reach it, or whether there is ground to hope 

 that it may some day be reached, Earthquakes are perhaps more 

 likely to yield information on this point, and as they can only, for 

 the most part, be the incompleted efforts of the molten matter to 

 force its way to the surface and find a vent, the two subjects may 

 be treated as one. Their direct connexion with eruptions is more- 

 over sometimes apparent, though not always, nor even very frequently, 

 for earthquakes are felt where there are no signs of any volcanic 

 activity having been manifested in recent times. In Japan, where 

 they have been particularly studied, no connexion can be traced, but 

 in Italy, on the contrary, the connexion is apparent. Earthquakes 

 are certainly due to movements of the earth's crust, and as these 

 are primarily of two kinds, either of elevation or subsidence, the 

 resultants are very different in kind and degree. Those due to sub- 

 sidence would probably be the more local, while those due to efforts 

 of the molten interior to break through would be propagated to great 

 distances. It would be impossible to attempt to enter into the 

 physics of earthquakes ; but I would merely say that the phenomena 

 of some are so wide-spread, and so independent of changes in the 

 nature of the superficial crust, that they appear as if they could only 

 have been propagated through the fluid or viscous substratum. The 

 smallest depth at which I have seen the focus of an earthquake 

 calculated is four miles ; this was in the vicinity of a volcanic region, 

 Vesuvius. Others have been determined at seven and ten miles. 



A peculiarity observed about earthquakes is that they are far 

 more prevalent in winter than in summer, and that they take place 

 more frequently when the barometer is low.^ This effect must have a 

 cause, and the only one apparent is that the diminished pressure of the 

 atmosphere admits of a corresponding expansion of the fluid interior. 

 The tension must obviously have been already extreme, for so small 

 a reduction of the pressure to produce disturbances at a depth within 

 the earth's crust that may be sensibly felt at the surface. But what 

 an idea this gives of the flexibility of the crust, and how thin it must be 

 to be acted on by so slight a cause ! Besides their connexion with 

 volcanoes, the direct connexion of earthquakes with underground heat 

 is apparent in many ways. One of the most curious examples of this 

 was the sudden and permanent increase in temperature of the waters 

 of the Bagneres de Luchon from 46° to 122° F., following the great 

 earthquake of Lisbon. Another proof that some classes at least of earth- 

 quakes are propagated through the fluid substratum lies in the fact 

 that mountain chains often, as in Japan, prove a complete barrier to 

 their progress, for the Eev. Osmond Fisher has shown, in his admir- 

 able work on the Physics of the Earth's Crust, the high probability 

 that mountain chains interrupt to a certain degree the continuity of 

 the fluid layer. Other instances of the influence of earthquakes on 

 thermal springs must no doubt have been observed, but I have not 



iSee J. Rofe, C.E., F.G.S., "On Colliery Explosions," etc., Geol. Mag. 1867. 

 Vol. IV. p. 106. 



1»ECAJ)E III. TOL. II. NO. IX. 26 



