45G 
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. H ere, 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 and 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, 2^th 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, 
