86 Transactions. 
respects for purposes of exact scientific inquiry, I trust that their discussion 
may lead to the adoption of instrumental means for recording such pheno- 
mena in future, as it is in this manner alone that sufficient accuracy can be 
obtained. 
In the first place I will explain, in a few words, the exact nature of the 
recently-experienced phenomena, concerning which there is a good deal of 
misconception. 
Notwithstanding the apparent rigidity of the rocks which form the crust 
of the globe, they are nevertheless truly flexible and elastic; and, for the 
propagation of earthquake shocks, those which appear to us to be most 
compact and stubborn, are really the most elastic and susceptible of rapid ` 
vibratory motion. 
The manner in which earthquake shocks affect the surface of the earth, 
and the secondary phenomena by which they are accompanied, is now well 
understood; but only a few of the many causes which may lead to their 
production are yet ascertained. 
Earthquakes occur, and perhaps originate, in every part of the earth’s 
crust; and, from the researches of Mallet, there is good reason to believe 
that the surface is, in some part or other, continually being subject to the 
jarring motion which they produce. Volcanic regions are particularly liable 
to them; but there they seem to be only local phenomena, that fail to pro- 
duce very distant effects. Volcanic energy has indeed been generally 
adopted as the cause of earthquakes; but applying the term to those forces 
by which masses of molten and chemically altered materials are heaped up 
on the surface of the land or poured out beneath the ocean, it is more 
probable that such convulsions are not originated, but only set loose, by the 
passage of waves of motion through the crust of the earth, which in their 
origin are quite independent of the local tension or constrained force which 
gives rise to the volcanic eruption. 
We are rather led to look on the passage of earth-waves as the normal 
state of things, depending, like the ocean tides, on cosmical causes exterior . 
to our planet. When their passage is interfered with they become percep- 
tible to our senses: when they interfere with and let loose pent up forces or 
tensions in some portions of the earth they lead to sudden convulsions, 
which in their turn give rise to secondary phenomena that produce the most 
terrific and appalling catastrophes. These great convulsions appear to 
occur, nearly in every case, at the bottom of the ocean, and where it has a 
profound depth at no great distance from land. 
The phenomena which attend such convulsions in the order they would 
appear to an observer on the shore of the neighbouring land, have been 
described by Mallet as follows :— 
