Mr Milne on Earthquake'Shocks felt in Great Britain, 93 



hangs so deep a cloud of mystery. Some philosophers think, 

 that, in the subterranean temperature of the earth, increas- 

 ing as it does about one degree of Fahrenheit for every forty 

 or fifty feet in descending from the surface, there is a per- 

 fectly sufficient cause for the outburst of volcanic fires and of 

 molten lava, which they derive from an intensely and per- 

 manently heated nucleus. Others, again, contend that, by 

 chemical agents alone, acting in certain parts of the globe, 

 the evolution of heat and its accompanying phenomena may 

 be accounted for : and this last class of philosophers is sub- 

 divided into two sections — one relying on the decomposition 

 of Avater, and the other on that of atmospheric air, penetrat- 

 ing down from the surface to the interior of the earth, and 

 there forming combinations w^hich give rise to these pheno- 

 mena. 



It is natural that there should be much vague and opposite 

 speculation, regarding the nature of forces which are them- 

 selves far beyond the reach of observation. It is only by 

 watching the effects of these forces under every modification 

 exhibited on the earth's surface, and especially by comparing 

 the phenomena, which occur (whether simultaneously or not) 

 in regions of the earth differing in geological structure, and 

 far apart from each other, that a knowledge of their true na- 

 ture can be acquired. 



It is in foreign countries, that the British geologist has 

 hitherto been in the practice of searching for and observing 

 the indicia of volcanic action ; — for it seems to have been 

 thought that the phenomena were unsatisfactory or unworthy 

 of attention, unless accompanied with eruption. But if, as is 

 now generally admitted, active volcanoes serve the purpose of 

 safety-valves, to give ready vent to the subterranean forces, 

 the effect of these forces on the earth's surface ou":ht to be 

 greater where no volcanoes exist. At all events, and even 

 though the forces themselves are in all places of precisely the 

 same nature, it is evident that in non-volcanic countries, their 

 mode of operation must be in many respects materially diffe- 

 rent. 



If these remarks be well founded, it is matter of regret 

 and reproach to British geologists, that, furnishing as their 

 own country does, frequent opportunities of observing the 

 occurrence and the operation of volcanic action, no at- 



