176 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
surface of one of the rough master joints in the set occupied by 
diabase dikes. Since these master joints were guiding planes for the 
intrusion of the dikes, no causal relation can be shown between the 
intrusive masses and these discoid joints. The structures also occur 
without any known relation to intrusive rocks, in the pelites formerly 
exposed on the grounds of Tufts College in Somerville. The cause 
of these fractures requires further investigation. 
The circular outline of the fracture is evidently accounted for by 
the fact that the wave of separation died out in essentially homo¬ 
geneous materials, hence the fracture spread to equal distances 
in all directions. Viewed in this way these discs may be regarded 
as whole joint-planes. If they are young or arrested joints, we 
should expect to find in massive strata similar circular fractures 
greatly exceeding in size these examples from the Mystic River 
section. The fact that these do not occur even in this area, indicates 
that the small discoid joints have perhaps a somewhat different 
origin from the elliptical joints. 
Other Occurrences of the Feather-Fracture. 
Feather-fracture and the coarse structure forming the joint-fringe 
may be observed on fractured surfaces of numerous rocks and 
other substances. Feather-fracture is a common phenomenon of split 
materials. The additional occurrences here considered are — 1, in 
coarse shales; 2, transverse joints of lava columns; 3, on columnar 
structure of basaltic rocks; 4, in felsites ; 5, granitites; and 6, in 
glass. 
Radial Shale Fracture. 
The structural distinction between shale and slate lies in the fact 
that in shale the fractures are not persistently parallel, but within 
a few inches or at least a few feet run together, the margin of 
one coalescing Avitli the next above or below, so as to divide the 
rock into more or less lenticular fiattish pieces. In slate, on the 
other hand, the fractures are more persistently parallel, and the rock 
is divided into plates or laminae of uniform thickness. 
The shale structure is generally parallel with the bedding, but 
may depart from it several degrees. The so-called “ slaty structure ” 
of many tilted sandstone beds in eastern Massachusetts is more 
closely allied to the shale fracture than to true slate structure. 
