WOODWORTH: FRACTURE SYSTEM OF JOINTS. 
169 
of the materials of which is lower than that of the bounding beds. 
The fracture therefore spreads to a greater distance from the point 
of origin along the length of the stratum than it does in a direction 
perpendicular to the bed. Were the fracturing force to be satisfied 
before the upper and lower surfaces of the bed were reached, we 
should expect to find a resulting circular area of fracture. The 
occurrence of discoid joints (see Plate 2 , fig. 7) lying within a 
single layer of rock confirms this view. 
The Joint-fringe. 
Beyond the edge of the joint-plane proper, there is frequently 
developed a set of planes, parallel to each other, extending upward 
from the upper and downward from the lower edges of the joint- 
plane into the surrounding rock. The dimensions of these planes 
are usually small as compared with the length and breadth of the 
joint-plane. For convenience I shall call them border-planes (b- 
planes), and the structure they form the joint-fringe. 
The border-planes of the fringe. These b-planes, where their 
relation to the joint-plane is most clearly shown, spring out from 
the edge of the joint-plane with which they are at first parallel, 
and proceeding outward turn gradually so as to be inclined from 
5 to 25 degrees or more to that plane. These b-planes usually 
lie above and below, in the layers bounding the bed cut by the prin¬ 
cipal joint, so that if we represent, as in Plate 1, fig. 6, the direction 
of the master joint in the intermediate bed by a straight line, the 
subsidiary joints of the fringe in the beds above and below will be 
represented by the short inclined lines, in which the direction is 
generally at an angle of 25 or more degrees to that of the principal 
joint. How far the greater joints in rocks are attended by these 
fringes of inclined joints, investigations have not yet fully shown; 
but it is evident that they occur, and it follows from this that in 
noting the direction of joints in the field, particularly where numer¬ 
ous small joints all of the same direction appear on the surface of an 
outcrop, precautions must be observed lest the direction of the minor 
fractures of a fringe be taken instead of the course of a main joint. 
This fringe of small joints also borders the convex edge of a joint 
where the joint-plane dies out along the middle of a stratum. 
Examples of this are shown on Plate 5 , in figs. 1 and 2. 
