170 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
Inclination of the b-planes: — The inclination of the b-planes to 
the joint-plane is greatest when taken farthest from the medial line 
of that plane. An exception to this statement occurs where the 
planes in the fringe spring out from the periphery of the joint-plane 
at a very sharp angle to it, frequently as high as 45 degrees, in 
which case the surfaces of the joint-plane and the b-planes are 
not conterminous at the point of union. The meaning of the 
inclination of the small planes will be more evident when the cross¬ 
fractures connected with them in the fringe have been described. 
Inclination of the fringe to the main joint-plane: —The b-planes 
in the fringe seldom lie in the same plane as that of the joint-plane 
proper, but depart from it at low angles of 5 or more degrees. 
Even when the surfaces of the fringe and the joint-plane are 
parallel, there is sometimes an offset or shoulder of hackly frac¬ 
ture, separating the two planes. Plate 2 , fig. 2, and Plate 4 , fig. 4, 
illustrate these relations of the fringe to the principal joint-plane. 
Direction of obliquity: —The direction of the oblique b-planes to 
that of the joint is probably of importance in the diagnosis of the 
stress phenomena which produced the fracture. The angle of this 
obliquity where it varies at all increases toward the outer limit of 
fracture. This obliquity may be said to be dextral or sinistral when 
the observer is looking down upon the upper edge of the joint 
placed in a vertical attitude. Observations are needed to determine 
whether or not all the border-planes in the fringe of an elliptical 
joint are parallel to each other. In the experiments with torsion of 
glass, small fractures apparently analogous to these oblique fractures 
in the fringe of a joint are formed at right angles to each other 
on opposite sides of the bar. The parallelism of the b-planes in 
the superior and inferior aspects of the fringe in the small joints 
in the Mystic River quarries would seem, then, to indicate that 
the fractures were not due to torsion. Where the obliquity has 
a definite relation to torsion, it would follow from this experiment 
that sinistral obliquity in the upper fringe of a vertical fissure is 
due to a right handed twist, and vice versa. 
Delation of the fringe to feather f racture: —It will be observed 
that the divisional surfaces in the fringe, in the examples which 
I have taken for a type, give rise to coarse lines of fracture 
divergent from the edge of the joint-plane. Towards this edge, 
the c-fractures diminish in size. Passing this edge, however, one 
sometimes finds the coarse distal markings of the feather-fracture 
