398 J. G. Goodchild—On “ Joints.’ 
apart as to divide what would otherwise have been a continuous 
mass of stone into separate blocks of an angular form. Four types 
of jointing may be recognized. 1. Prismatic jointing, such as gives 
rise to the divisional planes developed during the consolidation of 
certain rocks of metamorphic or of eruptive origin. Under this 
head is intended to be included the horizontal as well as the 
inclined joints observable in such rocks as granite, basalt, and 
others. 2. Shrinkage joints, such as are more obviously due to the 
contraction in bulk of the rock where they occur. The cracks 
developed in drying clay exemplify one extreme of this series; 
while the shattered and splintered character of certain dolomitized 
limestones, due to the change of dimensions of the rock in passing 
from simple carbonate of lime to dolomite, will serve as an example 
of the other. 3. Fault joints, or the prominent set of fissures 
commonly affecting rocks contiguous to great zones of dislocation. 
4, Superinduced divisional planes of the kind ordinarily affecting 
consolidated rocks of sedimentary origin. For the present purpose 
these will be distinguished as Rift-joints, and it is their mode of 
occurrence and their probable origin that it is herein proposed to 
discuss. 
The points of resemblance between joints and cleavage on the one 
hand and joints and faults on the other are often so numerous that it 
will be as well to state here some of their leading points of agree- 
ment and of difference in order to arrive at a clearer idea of the 
origin of the structure under consideration. 
Taking the case of stratified rocks, in order to simplify the ques- 
tion, we find that as a rule the downward direction of joints bears a 
definite relation to the planes of bedding, whether the rock itself is 
inclined from its native position or not. Joints are therefore older 
than the disturbances that produced the inclination of the rock. 
Cleavage, on the other hand, bears no such definite relation to the 
bedding planes, but cuts downward through the rocks at an approxi- 
mately uniform inclination, even where the rocks it affects happen to 
be flexed to every possible angle with the horizon. Cleavage is 
therefore posterior in date to the last great disturbance affecting the 
beds it traverses. Joints rarely, perhaps never, occur in less than, 
two sets, whose orientation is such as to cause them to intersect each 
other at large angles. Cleavage, in all but very exceptional cases, is 
confined to one direction. Joints intersect the bedding at approxi- 
mately equal distances apart, the distance varying according to the 
nature of the rock traversed; and the solid rock enclosed between. 
the joint planes exhibits, at the most, only an imperfectly-developed 
tendency to fracture across the bedding more readily in certain 
directions than in others. These last planes of weaker cohesion 
northern masons term the ‘ Bate,” and they play a by no means un- 
important part in the weathering of rocks, quite as much as in facili- 
tating the work of the stonemason. Joints occur in stony rocks of 
all kinds, quite irrespective of their composition, their lie, their 
place in the geological series, or their position on the earth’s surface. 
Cleayage produces a tendency to split in one direction only, and that. 
