TRANSACTIONS OF THE SECTIONS. 59 
sent General Secretary, Mr. W. Hopkins, who is so distinguished an ornament of 
this University. To the correctness of the mathematical reasonings employed in 
these researches no exception is of course to be taken, even by those who may 
withhold their assent from some of the conclusions arrived at. I profess my in- 
capacity to engage in the discussion of mathematical problems. Nevertheless, 
it es sometimes occurred to me to suppose that, however sound and legitimate 
may be the conclusions thus drawn from the premises assumed, they may still be 
imperfect or inadequate as conceptions of the truth, in consequence of the incom- 
leteness of the assumptions on which they are based. I shall not venture, even 
by a guess, to attempt to supply this defect. I only wish to regard the question 
as still an open one, thinking it possible that some condition or some agency may 
have been hitherto omitted from the speculation, of which no one has as yet, per- 
haps, formed even a conception. The researches already made may be admirable 
euides in all future investigations, and most useful in clearing the way for them; 
but it may nevertheless be dangerous to take the conclusion as so far established as 
to render future inyestigation unnecessary. 
There is one line of research, however, in pursuing which we may feel sure of 
the ground on which we tread, and that is the observation of occurrences which 
take place before our eyes, and of structures which each one may see and examine 
for himself. 
We have, in Earthquakes and Volcanos, the external symptoms of the action of 
the earth’s internal forces. What they do now, we may feel sure they were able 
to do formerly ; and we have no right to assume that they ever did either more or 
less within a given period than they have done during historic times. 
Volcanos drill holes through the crust of the earth, and eject lava and ashes 
through these holes. ‘These holes are often arranged in lines, as if they were con- 
nected with linear cracks in the earth’s crust. 
Earthquakes jar and shake the earth’s crust, throw its surface into transient 
wayes, and cause sometimes cracks and open fissures to appear at that surface, 
The largest of these fissures, however, are rarely more than a few miles in length 
and a few yards in width, and they appear rarely to leave any permanent traces on 
the surface, or to give rise to any of its more striking features. No one has ever 
yet pointed to any yalley or any glen, still less to any river-course, as haying been 
entirely caused by the gaping of the surface during any known earthquake, and in- 
dependently of subsequent erosion by running water. 
Mx. Mallet’s researches have given us the means of calculating the depth at 
which the impulse of an earthquake may originate. This Benth seems to be always 
proportional to the extent of the surface affected, from which it is obvious that in 
many cases a yery considerable thickness of the external envelope of the earth 
must have been traversed by these moyements. Supposing them to have a local 
origin, and to be caused by, or to he accompanied by, any considerable disturhance, 
either of flexure or fracture, in the solid or quasi-solid rocks at or about the centre 
of origin, it seems necessarily to follow that the amount of disturbance must lessen 
as we recede from that centre, in proportion to the thickness and extent of the 
matter oyer which it is diffused. ‘The tremblings and undulations, then, and the 
surface-cracks and fissures produced hy earthquakes are probably only the slight 
external indications of more intense but more local disturbance below. Great open 
fissures and gapings of the surface could only, as it appears to me, be caused by 
disturbances originating at a comparatively slight depth, where it is difficult to 
imagine any cause for them, and where, as a matter of fact, great disturbances never 
do seem to originate, 
In addition to the more conyulsive movements of the shocks, permanent eleya- 
tion and depression of the surface take place during earthquakes, and also to an 
equal if not greater extent by a slow gradual movement, unaccompanied by earth- 
quakes, and therefore not perceptible to our senses. These risings and sinkings of 
the surface are evidently the result of the upward or downward moyement of the 
whole thickness of the earth’s crust, whatever that thickness may be. 
_ Resting on considerations such as these, thus hastily sketched out, I am inclined 
to be bold enough to dispute the physical possibility, or at all events to deny the 
actual occurrence, at any time, of such surface manifestations of internal force as; 
