Mr. Hopkins’s Abstract of his Memoir on Physical Geology. 361 
seems almost impossible to conceive any efficient physical 
cause. On the other hand (still supposing the coincidence of 
the directions of joints and of lines of fracture), if we assume 
the formation of joints to have been posterior to the elevation 
of the mass, this coincidence will still remain to be accounted 
for. Our ignorance, however, of the process by which this 
structure may have been superinduced, will not at present 
allow us to do this. The fact must continue to offer a theo- 
retical difficulty, but one, I conceive, very different in its 
nature to that above stated, since it would appear, I think, 
probable that, taking a portion of the elevated mass bounded 
by adjoining fissures, the position of the joints subsequently 
formed in it should have some relations to the boundaries of 
that portion. The fact, therefore, of the coincidence (or rather 
parallelism) of direction above mentioned, while it presents a 
difficulty, does not seem to offer any @ priori objection to the 
theory which involves it. It is proper, however, to observe 
that though it should appear, for the reasons now stated, that 
a preference may generally be due to the theory which would 
assign the production of fissures to the elevatory force alone, 
we should by no means be justified in the rejection in every 
instance of that which attributes the directions of those fissures 
to the previous structure of the mass, and especially in those 
cases in which we fail to recognise distinct lines of elevation, 
or the usual relations between them and the lines of dislo- 
cation. And here it may be remarked as a striking fact, that 
the only mining district in this country in which there is any 
difficulty (as far as I have yet ascertained) in tracing these 
relations, is that in which, for independent reasons, it appears 
most necessary to recognise the influence ofa previously veined 
or jointed structure on the directions of its dislocations. I 
allude to the mining district of Cornwall. 
In the above reasoning I have assumed the accurate coin- 
cidence of the directions of joints and of lines of fracture, and 
it is important to observe that this accuracy of coincidence is 
essential to the theory which would assign the latter phzeno- 
mena to the prior existence of the former. A difference of a 
few degrees in the angular positions of the above lines would, 
if iehats established, be fatal to this theory, because, as I have 
already explained, although a fissure produced by an eleva- 
tory force would cross a line of less resistance under a certain 
condition, without change of direction, that condition cannot 
be generally satisfied when the angle between the fissure and 
line of less resistance is small, and in such case the fissure will 
be propagated exactly along the latter line. Observations on 
this point would therefore demand great care and accuracy. 
Third Series. Vol. 8. No. 48. May 1836. 20 
