402 J. G. Goodchild— On “ Joints.’ 
constitution admits of the rock as a whole yielding with more or less 
facility under lateral tension, the joints are usually very imperfectly 
developed or are absent entirely. 
The character of the bedding also affects the mode of occurrence 
of joints, for whereas in thickly bedded rocks of homogeneous and. 
compact nature joints usually go clean down, without sensible 
deflection, from top to bottom of each post (or individual bed), in 
such rocks as present much variety in that respect, there is a very 
marked tendency towards an interrupted arrangement of the joint 
planes, giving rise to what masons term “bonding.” In other 
words, instead of the joints extending uninterruptedly downwards 
through the strata, like the spaces do at the right or the left margin 
of these lines of print, the joints in one stratum will be found to 
underlie, or to overlie, an unjointed part of another bed of the rock, 
in the same way as the spaces between these words here and there 
underlie, or overlie, as the case may be, a continuous line of type. 
The cleat of a coal-seam affords another instance of the same kind, 
which comes out with much prominence where the coal is devoid of 
partings and is at the same time associated with shales, or with fire- 
clay. In this, as in the other cases cited, the development of the 
divisional planes is directly proportionate to the degree of resistance 
that any given bed offers to lateral tension. Where the rocks give 
out readily sideways, the layers slide over each other when they are 
stretched ; but where they are so constituted that they cannot readily 
change their dimensions in that manner, the rocks give way in other 
directions. 
In speculating upon the causes that have led to the formation of 
joints then, it appears to me necessary to bear the following consi- 
derations in mind :— 
(1) They affect every rock that has passed into the stony condition. 
(2) Their date of formation in any given case is approximately 
coincident with that of the consolidation of the rock they affect. 
(3) Their degree of development is inversely proportional to 
the lateral extensibility of the rock. 
(4) Their mode of occurrence under given conditions is uniform 
over the whole of the earth’s surface, and throughout rocks of every 
geological period. 
(5) They occur rarely, perhaps never, in less than two sets, 
whose respective orientations are such as to cause them to intersect 
each other at angles approaching to right angles, while their down- 
ward traverse is approximately perpendicular to the planes of bedi 
and has affected each bed independently. 
Taking these facts into consideration, it appears tolerably certain 
that while there can be no doubt about joints having been produced 
by disruption effected by mechanical means of some kind, simple 
torsion does not appear to be adequate to account for many of the 
facts in a manner that is altogether satisfactory. The subject needs 
complete reconsideration with a view to arriving at some explanation 
that shall be capable of application to all the phenomena yet known 
in connection with jointing. 
One such explanation occurred to me in 1872 while mapping the 
