WOODWORTH: FRACTURE SYSTEM OF JOINTS. 
175 
tlie joint-plane in whose fringe these marginal fractures lie was in 
the overlying rock which has been removed by quarrying or denu¬ 
dation. The same method of procedure should be employed in 
detecting the existence of joints of this type in blocks intended for 
masonry. 
Detection of veins :—Vein deposits are sometimes made in a 
gaping joint-plane and the attendant fringe. The examination of 
an exposed fringe-vein should be conducted as in the case of the 
joint alone. 
Discoid Joints. 
At a number of localities in the argillaceous strata of the Boston 
Basin, but nowhere more clearly shown than in the Somerville 
district, are small discoid fractures having a diameter varying from 
half an inch to two inches. The fractured surface may be divided 
into two fields, one forming a narrow rim surrounding the other. 
The large central area consists of b-planes and c-fractures, the latter 
radiating outwards from the center so as to expose wedge-shaped 
areas of the former. The appearance of one of these discs is shown 
on Plate 5 , fig. 5. The outer rim is conchoidal in its habit of frac¬ 
ture. The warped surface thus formed may in some specimens 
be likened to the rim of a soft hat curled up behind and pulled 
down in front. (See Plate 2 , fig. 5.) This rim of conchoidal 
fracture is in other cases curved upward like the edge of a saucer, 
so that there is a close resemblance to the ball-and-socket jointing 
of the prismatic columns of igneous rocks. 
That these fractures are true joints seems to me evident from the 
homology of their structure with the joints already described. They 
are obviously not impressions of an organic nature. Though fre¬ 
quently stained yellowish brown with hydrous oxide of iron, this 
discoloration is a slight deposit on the b-planes, laid down before 
the c-fractures were made. The same discoloration occurs in the 
Mystic River quarries on the b-planes in the fringe of typical ellip¬ 
tical joints. The center of these discoid joints is not different from 
the rest of the rock in texture so far as I have been able to observe; 
with one exception, in which there is a small ring or areole of unde¬ 
termined origin. 
In the Mystic River quarries groups of these discoid joints lying 
approximately in the same plane have been seen exposed on the 
