WOODWORTH: FRACTURE SYSTEM OF JOINTS. 
171 
merging into the b-planes and c-fractures of the fringe. It, there¬ 
fore, seems probable that the feather-fracture is simply a minute 
series of fractures divisible into b-planes and c-fractures, the size 
of which and the interval between the respective sets of which 
become so small as not to be detected by the unaided eye. In 
the inner stages of the feather-fracture the surface of the joint- 
plane assumes a granulated appearance. The fractures correspond¬ 
ing to the b-planes are almost parallel with the main plane of jointing. 
I was so fortunate as to find one specimen in which the surface 
of the joint is traversed by combinations of coarse b-planes and 
c-fractures. (See Plate 4 , fig. 3.) 
Feather-fracture in the fringe :—Rarely feather-fracture has 
been observed on the b-planes in the fringe, showing that these 
smaller planes are likewise combinations of fractured surfaces (Plate 
1,%. 9). 
Cross fractures :— It will be observed in the plates accompanying 
this paper that the successive b-planes form oblique plates of rock 
which are traversed by hackly fractures passing between the b-planes. 
These cross-fractures are of two types. In most cases, the oblique 
plates or bridges of rock between b-planes break across about midway 
of their length and form straight medial cross-fractures. Another 
mode of fracture arises from the continuation of the rupture from the 
end of one b-plane, in a curved surface, to the next adjacent b-plane, 
as shown on Plate 2 , figs. 4, 9, 10. This curved lateral cross-frac¬ 
ture results in the formation of unlike joint-blocks. One of the 
opposed faces in contiguous blocks has a fringe of grooved surfaces; 
the other side of the joint is a block with convex surfaces, from the 
reentrant angles of which the b-planes extend obliquely into the 
joint-block. (See Plate 4 , fig. 4.) 
The straight medial cross-fracture springs out from the edge of 
the joint-plane and widens towards the outer edge of the fringe 
with the turning of the b-planes. This hackly fracture is generally 
more recent than the splitting which produced the b-planes. This 
is shown in some of my specimens by deposits of a limonitic crust 
covering the b-planes but not found on the cross-fractures. These 
c-fractures are inclined also at considerable angles to the joint-plane. 
This difference of age in the b-planes and the c-fractures shows that 
there was a time in the history of at least certain joints when the 
fringe if not the whole divisional plane consisted of a series of small 
oblique joints arranged en Echelon as in the diagram, Plate 1, fig 7. 
