WOODWORTH: FRACTURE SYSTEM OF JOINTS. 
173 
In the great joints which give rise to the wall-like faces of the 
trap at West Rock, New Haven, where the joint-planes exceed 
100 feet in vertical diameter, and where the border planes of the 
joint-fringe are from 15 to 20 feet long, this interval is often 
greater than 3 feet near the lower contact of the sill. 
Him of conchoidal fracture: — Outside of the fringe, orbordering 
the joint-plane when the fringe is wanting, there may be developed 
a rim of conchoidal fracture. Structurally the rim is a compound 
surface of fracture. It tends to be inclined to the joint-plane 
and to the joint-fringe. It is well developed in the discoid-joint 
(see below). 
Summary of preceding structures —If, to the whole unabraded 
surface of fracture above described, we give the name joint, the 
recognizable parts of that structure, some of which are characteris¬ 
tically present in typical cases, will present the following taxonomic 
scheme. (See Plate 1, fig 10.) 
Joint 
joint-plane 
i uniaxial 
feather-fracture < or 
f twinned. 
(_'perpendicular to) 
I 
( elliptical 
edge < or 
( circular. 
{surrounded by) 
< 
S 
dextral 
joint-fringe < 
border-planes ( or 
( sinistral. 
(perpendicular to) 
cross-fractures 
( straight medial 
} or 
( curved lateral. 
{surrounded by) 
rim of conchoidal fracture. 
In this scheme “ perpendicular ” means approximately at right 
angles; and the rim of conchoidal fracture may come next to the 
joint-plane, where the fringe is imperfectly developed or wanting. 
