J. G. Goodchild—On “ Joints.”’ B97 
sponding to that of the perennially frozen ground, it would be 
perennially frozen also, and its further progress through the fissure 
arrested ; but reflection showed me that the warm air of the cave 
must have been continually rising through the fissures, and unless 
these were hermetically blocked must have kept their faces, and the 
material near thereto, unfrozen, and so allowed its entry to continue 
every summer. 
IiJ.—Jornts. 
By J. G. Goopcuitp, of the Geological Survey. 
F the various theories that have been advanced from time to 
time to account for the origin of jointed structure in rocks, the 
one that seems to find greatest favour amongst geologists of the 
present day is that put forth in M. Daubrée’s Kiudes Synthétiques de 
Géologie Expérimental, vol. i. p. 300, e¢ seq., wherein their formation 
is ascribed to the effects produced by torsion. No one can read 
the account of M. Daubrée’s experiments without feeling convinced 
that the deductions he has drawn will enable us to account for the 
origin of many forms of joints in a manner that leaves little or 
nothing to be desired. The mere fact that some of the foremost 
leaders of geological science at the present day hold views on this 
subject practically identical with his, will, doubtless, be generally 
regarded as a sufficient reason for accepting M. Daubrée’s theory 
as one that has supplied a definite and satisfactory answer to the 
much-vexed question of the origin of joints in general. 
But in testing the validity of this theory by a large number of 
facts, one now and then comes across a case that does not seem to 
admit of such an explanation in a way that is altogether satisfactory. 
Such cases are, doubtless, rather to be regarded as exceptional, and 
as exceptions, they will perhaps be regarded as going to prove the 
rule itself. But they have to be accounted for somehow, whether 
they conform to a generally-accepted theory or not; and in attempt- 
ing an explanation of these apparent exceptions we may chance to 
light upon some modification of the original view that may prove 
of sufficiently wide application to cover all the known facts relating 
to the subject under consideration. 
In the present paper it is proposed, after detailing some facts 
relating to joints of the ordinary kind, to direct attention to a few 
instances of this exceptional kind, and then to offer for the con- 
sideration of the readers of the Grotocican Macazine one such 
modification of M. Daubrée’s theory as appears in some respects to 
be more in accordance with the facts yet made known. 
Joints vary so much in their mode of occurrence that it is difficult 
to formulate any definition that will apply to all cases and yet be 
exact; but the following may suffice for the purpose at present 
in view. 
Joints may be defined as the divisional planes that cut in two or 
more directions approximately perpendicular to the outer bounding 
surface of consolidated strata in such a manner and at such distances 
