WOODWORTH: FRACTURE SYSTEM OF .JOINTS. 
179 
Felsites: —The joint-fringe with straight medial cross-fractures 
occurs typically developed in the banded and contorted felsites on 
Marblehead Neck. The edge of the joint-plane is a tolerably 
straight or slightly curved line beyond which the interval between 
the imbricated b-planes rapidly increases. The c-fractures are 
hackly surfaces, evidently the last series of ruptures in the formation 
of the joint. B-planes with the cut off are common over the faces 
of the principal joint-planes. Many of these c-fractures are so fresh 
as to indicate that the actual formation of the completed fracture 
surface of the principal joint was not accomplished until the side of 
the ledge fell away or was forcibly removed. 
Hornblende granitites : — The joint-fringe also occurs typically 
developed in these rocks, showing that the formation of these planes 
is a property of brittle solids and is independent of the material and 
texture of the rock. 
Glass: — That the feather fracture and attendant marginal fringe 
O o 
of joints are developed in glass when it is shattered was first pointed 
out to me by Mr. F. C. Schrader. The plate of glass was about one 
quarter of an inch thick, and had been shattered by a blow into long 
slivers in which the fractures were curved. Several of these some¬ 
what divergent fractures cut one surface only and did not go through 
to the farther side of the glass. In the case of the dissevered 
surface, it was obvious that a layer about 1 mm. thick was cut by 
numerous small joints having a considerable angle with the principal, 
deeper lying, curved fracture. These planes were subparallel. 
The feather-fracture was definitely developed along the middle 
portion of the plane of fracture, with an axis off from which pro¬ 
ceeded plumose lines becoming perpendicular to the margin of the 
fracture. These feather-fracture lines diverged in the direction of 
splitting of the glass. 
REMARKS. 
Significance of the fracture system of joints in the interpretation 
of the larger fractures of the earth's crust: —The examples cited 
in this paper are all fractures on a small scale. Here and there 
in quarries the fringe of large joints may be seen on a scale 
many times that of the specimens on which these studies have been 
