J. G. Goodchild—On “ Joints.” 399 
to an almost indefinite extent, equally throughout rocks of particular 
constitution, and it does not affect the associated strata whose con- 
stitution happens to be different. In the British Islands it is 
confined, perhaps exclusively, to Older Paleozoic strata; and 
although jointing may be found all the world over, the geographical 
distribution of cleavage is irregular, and does not appear to be 
reducible to any law. Taking the case of the Older Paleozoic strata 
of the Basin of the Solway, for example; the rocks lying to the 
south of a line ranging approximately EH. and W. through Cross Fell 
exhibit a tendency to cleavage increasing in degree up to a certain 
point as the rocks are traced southward; while to the north of that 
line cleavage gradually becomes less and less perfectly developed, 
until, as has long been known, it disappears almost entirely on the 
Scottish side. Yet the evidence of intense compression is much more 
marked where the cleavage is imperfectly developed than it is to the 
south of the zone referred to; even though the lithological character 
of the rocks affected may be identical in both cases. It is instructive 
to compare with this the uniform distribution of joints within the 
same area. It is likewise instructive to note, in such cases as that 
just referred to, that whereas the inclination of the cleavage planes 
varies to the extent of only a few degrees on either side of the 
vertical, save in exceptional cases, due to an easily explained cause, 
the joint planes are evidently as much affected by disturbances as 
are the planes of bedding themselves. On the other hand, joints are 
not more fully developed in the case of rocks that have undergone 
much plication or twisting than they are in rocks that show no 
evidence of having been disturbed at all. In other words, the extent 
or degree of jointing is not commensurate with the amount of dis- 
turbance the rock has undergone. 
It would seem therefore that cleavage represents the effects of 
some special causes, which have affected particular areas only ; while 
jointing results from some general causes, successively brought into 
action at every known period of the earth’s history, and affecting all 
parts of the earth’s crust alike. 
The close relationship between joints and faults has frequently 
been the subject of previous remark ; but as I propose to bring for- 
ward a few observations upon the origin of faults in a succeeding 
communication, I shall do no more than refer to it now. 
That forces of a mechanical nature must have had an important 
share in the production of joints is rendered tolerably evident in 
many ways. One of the most striking of these is that of the clean 
cut fracture so often observed in cases where joints traverse compact 
breccias, or conglomerates, cutting slick through everything that 
comes in their way. It is common to find, amongst other constituents 
of such rocks, a big lump of quartz sheared in two by a joint without 
appearing to be in other respects in the least degree disturbed. The 
compact conglomerates of the Roman Fell Series, as well as the 
Brockram of Cumberland and Westmorland, the Hertfordshire Pud- 
ding-stones, and others, have long been known as presenting very 
remarkable instances of this kind. 
