166 PROCEEDINGS : BOSTON SOCIETY NATURAL HISTORY. 
planes as I shall show below. On the typical joint-planes of the 
Mystic quarry district, feather-fracture is a delicate tracery of 
feathery lines diverging from a roughly outlined axis which trav¬ 
erses the face of the joint-block in a plane parallel with the stratifi¬ 
cation. (See Plate 1 , fig. 1 ; Plate 4 , fig. 1.) When the axis of the 
feather-fracture departs from this plane, it becomes sinuous. In the 
massive rocks, it is characteristically sinuous. 
Feather-fracture should not be mistaken for slickensides. Feather- 
fracture disappears on joint-planes when the joint-blocks are slicken- 
sided. The preservation of feather-fracture depends upon the joint- 
blocks moving away from each other in a direction transverse to the 
surfaces, so that feather-fracture joints usually gape. There may be 
actual faulting, however, without slickensiding. The criteria of 
faulted surfaces are linear grooves and raised lines, with polished 
faces, together with fault-breccia. The criteria of feather-fracture 
are divergent lines and granulated surfaces, without polish, and the 
passage of the minute planes and hackly fractures of the middle of 
the joint-plane into the coarse structure of the same type in the 
joint-fringe. (See p. 169.) 
Relation to percussion rays : — Very similar lines to feather- 
fracture seen upon the surface of chipped Hakes and upon quarry 
spalls are familiar to archaeologists, to whom they are known as 
“radiating lines” or “percussion rays.” (See Plate 5 , fig. 3.) 
These percussion rays proceed from the point of origin of the 
fracture,— the “ bulb of percussion,” where the block is struck,— 
diverging outward so as to traverse at right angles the billowy sur¬ 
face of the concentric lines and slopes of the conchoidal fracture. 
These rays diverge in the direction of splitting; and their distal 
margin is therefore concave to the point of origin of the fracture. 
If these percussion lines are genetically the same as the lines in 
feather-fracture, their convergence in feather-fracture, therefore, 
furnishes an index to the point or area of origin of the fracture on 
which these splitting figures are displayed. 
Nature of percussion rays and feather-fracture: — Percussion 
rays and feather-fracture, where the latter is well developed for 
examination, are seen to be made up of two similar sets of fractures : — 
1st. A set of small narrow planes of fracture, approximately 
parallel with the main fracture or joint, the exposed face of these 
small planes increasing in width with the divergence of the so-called 
percussion rays or lines. 
