WOODWORTH: FRACTURE SYSTEM OF JOINTS. 
167 
2d. A set of rough, hackly fractured surfaces, more or less 
perpendicular to the first named and to the main fracture surface, 
forming minute steps from one minor plane to the next above or 
below. Examination shows that these fractures are due to the 
breaking of the oblique thin plates of rock formed by the minor 
divisional planes of the first set. 
Feather-fracture as exhibited on joint-planes in stratified rocks 
frequently differs from the rays of percussion in being divergent from 
an axis instead of from a point; in being developed about an area 
which obviously is not physically analogous to the bulb of percussion. 
This last named phenomenon is the criterion of the chipped flake. 
Feather-fracture is the criterion of splitting in rocks. 
The axis of feather-fracture: — The divergence of these lines of 
splitting from an axis rather than from a point is, it is hoped, sug¬ 
gested in the term feather-fracture. This axis may be, as stated 
above, either straight or curved. The plumose lines on the right 
and left may vary in length. I have not observed in them a 
tendency to divergence from a secondary axis, except where such 
axis was developed upon a plane inclined to the main plane of 
fracture and in the marginal planes of the joint. Toward the margin 
of the joint-plane these side lines tend to become perpendicular to 
the edge of the joint. This edge where the joint cuts the stratum 
perpendicularly is usually determined by a change of texture in the 
rock depending upon stratification. Usually the lines and planes of 
the feather-fracture are most distinct near the margin of the joint. 
In many instances in joints, the structures are so closely set that to 
the unaided eye the whole resembles mere granulations arranged in 
a plumose manner. The axis, where a straight line, is not usually in 
the middle of the stratum. Owing to this fact there is great varia¬ 
tion in the length of the plumose structure to the right and left. 
Where the axis of feather-fracture appears at a given horizon in a 
stratified block on one joint-plane, it is apt to occur at the same level 
or in the same plane on the opposite side of the joint-block. This 
suggests that the texture of the rock on a given plane of sedimenta¬ 
tion has determined the position of the axis. There thus arises an 
axial plane in a joint-block. 
Fine examples of feather-fracture showing this axis may be seen 
on the stone front of St. Peter’s Episcopal Church on State St., 
Albany, 1ST. Y. 
•/ ' 
Reversed feather-fractures on the same plane :—A rare occur- 
