J. G. Goodchild—On “ Joints.” 401 
range of variety in both their lithological character and their angle 
of inclination. The joints traversing the limestone extend down- 
wards, as is almost invariably the case, at right angles to the planes 
of bedding, and the action of weathering has shown (as many a long 
day’s work in connexion with them has given me very good reason 
to know) that the jointing has been developed in these rocks in 
a very perfect and regular form. There are many instances of 
fissures extending uninterruptedly downward through a distance of 
several hundred feet, where the downward flow of surface-water 
has fretted away the rock along the joint planes, which could hardly 
have been the case had not the jointing been, on the whole, directed 
vertically downwards with a certain amount of regularity. ‘The 
joints traversing the older set, on the contrary, lie at every possible 
angle, according to the varied dip. ‘That is to say, that in the over- 
stepping series in the case of an unconformity, the joints have been 
formed quite independently of the joints in the rocks beneath, and 
nothing more than a local and clearly accidental connexion can be 
traced between the newer set above the unconformity and the older 
set below. As the rending of the higher sheet of stony matter must 
(if I understand the theory above referred to) have proceeded from 
below upwards, it is difficult to understand how it is that the 
forces that produced the upper set of joints have not affected the 
immediately subjacent rocks in the same way. ‘To put the difficulty 
in another light, seeing that a series of well-developed planes of 
weak cohesion were already in existence before the joints above the 
plane of unconformity were formed, one is at a loss to understand 
how it is that these older joints have not guided the direction of the 
newer fractures in the same way as happens with faults under like 
circumstances. As it is, each set of joints seems perfectly independent 
of the other. 
Another fact having some bearing upon the present question is 
that joints are as well developed in strata left undisturbed in their 
original position as in the same beds where they are bent into folds. 
Joints are stretched a little wider apart perhaps, under these last 
circumstances, but that is about all the difference. 
The varied development of joints affecting any one group of strata 
whose several members differ much in structural or in lithological 
characteristics, is another point to be considered in connection with 
the origin of joints. A good instance is afforded by the Upper 
Yoredale strata in North-west Yorkshire, described by Phillips in his 
“Geology of Yorkshire,” in the same connection. In these the 
calcareo-siliceous members locally characteristic of this part of the 
Yoredale series are traversed, in many instances, by a system of neat, 
fine, joints whose mode of occurrence causes the rock to be divided 
up into blocks of almost geometrical regularity of form. As the 
succession changes, something more than a mere difference in the 
degree of regularity of jointing may be observed. Joints of some 
kind or other traverse the limestone below, and others, less regular 
still in their mode of occurrence, affect the sandstone beneath that ; 
but when we come to the shale bands or to any other rock whose 
DECADE II.—VOL. X.—NO, IX. 26 
