456 



THE GEOLOGIST. 



have the same origin. An earthquake of this latter class I believe, with 

 Mr. Mallet, to be an " incomplete effort to establish a volcano ;" that is, the 

 outward escape of pent-up volcanic heat. 



In many volcanic mountains we actually see the rents, the " snap and 

 jar " of whose rupture through the solid rocks forming the mountain's side 

 occasioned the earthquake. For instance, during the eruption of Vesuvius 

 in 1860, a violent earthquake-shock was felt along the southern base of the 

 mountain, and a crevice was seen to have opened through its flank behind 

 Torre del Greco, radiating from the side of the eruptive explosions down 

 to the sea, the coast of which was permanently elevated some two feet along 

 a considerable distance. In some of the violent eruptions of the volcanos 

 of Iceland, and also of the Pacific Islands, the mountain has been seen to 

 be split across from top to bottom by such shocks. 



The superficial fissures and changes of level which are often observed to 

 accompany earthquakes not attended by outward eruptions, testify to the 

 occurrence of some violent fractures and dislocation having taken place at 

 some depth beneath. Here, then, we have a vera causa, seen and known 

 to be at work in many instances, quite sufficient to explain the phenomena 

 of those where the cause is not so apparent, owing probably to the deeper 

 position of the point at which the shock originates. Is it not then, I ask, 

 the most reasonable theory to refer the earthquake to the same primary 

 cause as the volcanic eruption, namely, " the sudden expansion of some 

 deeply-seated mass of mineral matter, owing to increase of temperature or 

 diminution of pressure ?" (See p. 296 ; Volcanos.) 



If we suppose the heated matter below the crust of the earth — of the ex- 

 istence of which (at least throughout the great bands of volcanic and seis- 

 mic disturbance) we have positive ocular proof in its frequent eruptions — 

 to be (as of necessity it must be) exerting a continual upward pressure 

 against the overlying rocks, and creating in them a violent tensile strain, 

 it is certain that any diminution, however slight, in the amount of pressure 

 above them — even the sudden lightening of the atmospheric pressure alone — 

 may give occasion to the yielding of the cohesive force of the rocky crust, 

 and its consequent snapping aud jarring fracture, to which I attribute 

 every earthquake. Thus is explained the more frequent occurrence of 

 these phenomena at the periods of the Autumnal Equinox, and also when 

 the moon is at the meridian of the locality affected, as shown in the tables 

 of Mr. Perrey and Mr. Mallet to be the fact. 



We know that the solid crust of the earth is, and has been from the 

 earliest geological periods, continually undergoing oscillatory movements 

 of elevation or depression. These must have been always accompanied by 

 the fracture and Assuring of its rigid component rocks, at great depths no 

 less than near the surface. Do not these movements correspond with, and 

 amply explain, the frequent occurrence of earthquakes, which are precisely 

 the kind of phenomena we should expect to experience from such sudden 

 and violent snapping and rending of the rocks beneath us ? I content my- 

 self with this explanation of the cause of earthquakes, and think it quite 

 unnecessary to resort to any other, such as terrestrial electricity, mag- 

 netism, crystallization, the breaking-in of the roofs of imaginary subter- 

 ranean cavities, or the condensation of vapour evolved from submarine 

 volcanos ; to which last theory Mr. Mallet, as I think, unnecessarily resorts. 



I remain, Sir, your very obedient servant, 



G. POULETT SCEOPE. 



London, 20th November, 1863. 



[The late date at which Mr. Scrope's letter arrived, precludes the possibility of my 

 noticing at any length his very valuable communication. The experience of Mr. Scrope, 



