WOODWORTH: FRACTURE SYSTEM OF JOINTS. 
165 
which the principal plane of fracture is formed, and the peripheral 
aspects of the joint including its outline and the structures which 
mark its boundaries in the surrounding rock. This paper deals with 
these phenomena, mainly as they occur in stratified rocks, and 
incidentally with similar fracture features found in certain igneous 
rocks. 
PERSONAL OBSERVATIONS. 
Observations on joints in the fine-grained pelites of the Mystic 
River quarries : — A cursory examination of the joint-planes in the 
Mystic River quarries shows that these secondary divisional planes 
differ as regards the form of their surface. Some of the larger 
joints are wavy, the undulations corresponding to alternations of 
hard and soft layers the joints being perpendicular to the stratifica¬ 
tion. Such joints frequently show slickensides, as noted by G. F. 
Becker (’94), but I believe the rubbing to be largely secondary. 
Other joints are more or less ragged. Even those which approach 
nearest to perfect planes exhibit rugosities to the eye or to the touch. 
Of these latter features, there is a complete series from apparently 
systemless granulations of the surface to regular lines, and finally 
a system of minor planes. Smooth and polished surfaces occur, but 
they are mainly phases of the central area of a joint-plane which 
has been faulted. 
The Joint-plane. 
As is fully set forth below, a joint consists of definitely arranged 
parts. Of these the central, most conspicuous, and generally the 
sole recognized indication of a joint, is the joint-plane. It is pro¬ 
posed to describe in detail some of the macroscopic features of this 
approximately plane surface as they occur in the Mystic River 
region and numerous other localities. These features pertain mainly 
to a structure which may be termed feather-fracture in contradis¬ 
tinction to conchoidal-fracture. 
Feather-fracture : — The granulations of the surface of a joint- 
plane when organized in lines take on the form which I have 
become accustomed in field work with students in Harvard Univer- 
sitv to call “feather-fracture.” Feather-fracture is not limited to 
true joint-planes but is traceable on the surface of other divisional 
