98 Mr. Mautet on the Dynamics of Earthquakes. 
the earth. That earthquake shocks often come from profound depths is in a high 
degree probable; and while down to a certain depth we may expect to find the 
density and elasticity of the earth’s crust continually increasing, below this again, 
we must suppose the mineral masses in a more and more softened or even pasty 
condition, as they approach the lower fluid region, and hence possessed of lower 
elasticity. While, therefore, we cannot draw direct conclusions as to the time of 
transit of the wave in the rocks thus circumstanced at profound depths, from its 
time of transit in the solid rocks or superficial deposits of the surface, we may 
reasonably expect to derive information as to some of the physical characters and 
molecular condition of the deep rocks themselves, by comparing observations of 
the actual time of wave transit of natural shocks, coming from great depths, with 
that of natural or artificial shocks traversing at the surface or near it. 
On the other hand, when the modulus of elasticity has been determined for 
theprincipal rocks, at various temperatures, augmenting up to their points of fusion, 
and the same data have been obtained for them in a fluid state, we shall be in a 
position to demand assistance from the mathematician in determining the com- 
plex conditions of horizontal and vertical wave motion in a compound mass, solid 
at the surface, and increasing in density and elasticity down to a certain depth 3 
below this gradually becoming a pasty semi-fluid mass, with probably still in- 
creasing density, but diminishing elasticity ; and finally becoming a dense elastic 
liquid susceptible of fluid wave movements, at still profounder depths. 
Such a question can scarcely be attempted after the data, already alluded to, 
have been obtained, without our deriving some additional knowledge as to the 
constitution of the interior of our planet. 
In the progress of this inquiry, and in consulting very many accounts of 
earthquakes, one thought has been constantly suggested to me, which, although 
not directly belonging to the subject of this paper, may be very briefly noticed. 
While every part of the earth’s surface appears occasionally liable to earthquakes, 
and while volcanic countries are peculiarly so, though by no means remarkable for 
being visited with those of greatest violence, the origin, or centre of disturbance, 
of almost all the greater earthquakes appears to be beneath the sea, and at consi- 
derable distances from active volcanoes, as already observed. At the same time, the 
circumstances of the great sea wave seem to indicate that the centre of disturbance 
is seldom, if ever, very distant from the land. May it not, then, happen that the 
