ON THE FACTS OF EARTHQUAKE PHZNOMENA. 57 
through a formation of rock, bearing vertical or inclined plates of water, each 
of these will be powerfully compressed at the moment of the passage of the 
shock, for it is by the compression of the water only, that the shock itself can 
be transmitted onwards to the rock beyond the plate of water; and as fluids 
transmit in all directions equally, any pressure communicated to them in one, 
so the water in each such plate will press upwards at the moment of the 
shock, and will fly upwards, because in this direction resistance is least ; and 
as the pressure, though but for an instant, and even often acting through a 
very restricted range, yet acts on an enormous surface, namely, on perhaps 
the whole length and depth of the water plate, each amounting to miles, at 
the same instant, its total effect, guoad the volume of water displaced, is very 
great, for it is as though a piston of very short stroke but of enormous surface 
suddenly expelled water through a comparatively very small aperture. But 
furthermore, the spouting fluid has acquired a certain velocity which it does 
not lose at once, and has further an elasticity of its own, as well as the solid 
walls that compress it ; hence it spouts higher, and a rather greater volume 
of water is expelled from the fissure than is due merely to the diminution of 
its capacity by the passage of the shock and to the speed with which this is 
effected. Shocks in a vertical direction will affect such plates of water, if 
vertical or nearly so, precisely in the way already described for open cylin- 
drical wells. 
Proceeding now to the case in which the water lies in a water-bearing bed or 
in several, of sand or gravel, overlaid by rock or by clay, or any other im- 
pervious material: if the direction of shock be horizontal, as before, the 
water in such reservoirs will, by its inertia, oscillate first in the opposite di- 
rection to that of the shock, and then again in the same direction as the 
shock moves; and if in either of these directions its bed crop out to the 
surface, or it can find vent to it any other way, it may suddenly emerge in 
vast. volumes, and from the principle just alluded to, of quaquavérse pressure, 
such pourings out of water are not confined narrowly, to the line of direction 
of shock or its opposite, but may occur at any angle to it, laterally, or up- 
wards, or downwards. 
But let us now take the case in which a country underlaid by such a 
water-bearing bed or beds is exposed to a vertical shock or one nearly 
vertical. Here, on the principles already stated, every open well, or natural 
fissure or duct communicating with the water-bed through the impervious 
strata above, will spout out volumes of water; and if there be considerable 
tracts over which there are no such artificial or natural vents, or whose 
combined areas are insufficient to ease off the sudden pressure of the water 
upwards, it may break through the retentive stratum above, whether of rock 
or of clay, &c., at the points of least resistance, and there spout out where 
water had never been known before, and may bear with it volumes of gravel, 
sand, and mud from the beds below. 
Now these were just the conditions of the great Calabrian plain, and of that 
of Lisbon and of Port Royal. In Calabria, as we have seen, a vast deposit 
of clay forms the surface of the plain, described by Dolomieu as consisting of 
‘a stratum of vegetable earth, argillaceous, black or reddish, very strong, 
very tenacious, and from four to five feet in thickness,” beneath which lie 
various formations resulting from the decomposition of granite, and under 
this “ἃ white micaceous clay rather unctuous and ductile ;’ and beneath all, 
the deep bed of sand and scarcely coherent sandstone, which he informs us 
is a water-bearing stratum in most places, and to which he says “the roots 
of the trees penetrate to a great depth in search of the humidity always 
contained in the lower part of the sand.” 
Sir William Hamilton also describes the “swampy plain of Rosarno” as 
