WOODWORTH: FRACTURE SYSTEM OF JOINTS. 
181 
AB. B ut instead of this effect, it produced a series of parallel lines 
oblique with this course. We can conceive of such a system only 
JL %J 
on the ground that there is a tendency to fracture in a certain 
direction; and the force is applied obliquely to the lines of easiest 
fracture.” 
In the case of the oblique marginal fringe fractures of joints, there 
is some structural reason for the right or left handed obliquity of the 
b-planes. The outer portion of the b-planes frequently tends to be 
inclined 45 degrees to the direction of the joint-plane. If these 
small planes in the fringe are parallel with some preexisting cleavage 
structure in the rock, and the joint-planes form at angles of 45° to 
them, there may result two sets of parallel joints. Some specimens 
of joint-blocks show this relation, but my studies have not convinced 
me that the parallelism of the planes of the joint-fringe in any 
locality is at all so closely persistent. (See Plate 2 , fig. 11.) 
In the case of the curved fringe fractures, there is a likeness in 
long curved lines of volcanic islands skirting the western shore of 
the Pacific Ocean. (See Plate 3 , fig. 1.) These curves are on a 
large scale to be conceived as the volcano marked fringe fractures 
of a more uniform surface of fracture at a greater depth. The limit 
of downward fracture in rocks is estimated at probably 5 miles, 
though possibly 30 or even 50 miles (Lane, ’94). In the examples 
available for study the width of the fringe is usually less than that of 
the main fracture, and is commonly a fractional part of that dimen¬ 
sion. Unless these curved fractures of eastern Asia merge rapidly 
into a more general line of fracture, we must suppose that the 
fracture curves die out downward in the zone of flowage without 
such mergence. The convexity of the curves towards the ocean, 
well marked in each of the Asiatic curves, is well shown in other 
chains of volcanic islands, by the concavity which they present to 
the nearest continental mass, as in the case of Australia, Borneo, 
and the Sun da Islands off southern Asia, the West Indies off South 
America. The reason for this system of fractures probably lies in 
the torsion of the continental monoclinal fold lying; between the 
Asiatic plateau and the Pacific Ocean bottom. The resemblance 
to the curved joint fringe is suggested here more to show the 
need of extended studies of this neglected structure than confi¬ 
dently to assert an explanation of ranges of fracture marked out by 
volcanoes. But there is one striking example of analogy to the 
ideal straight medial cross-fracture system deserving attention. 
