178 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
cause quelconque. Je n’ai pas encore obtenu des correspondants 
pour toutes les formes, il est vrai, et je ne puis expliquer la forme 
spirale que traverse la roche; mais je n’en doute en aucune fagon, la 
solution definitive du probleme par la voie purement mecanique 
n’est qu’une affaire de temps.” 
Certain examples of discoid joints in the Mystic River pelites 
have many points in common with Spirophyton and its allies. 
Further study of large collections will, I believe, show that some 
forms of these so-called fucoids are due to the fracture of the rocks 
in which they occur. (See Myrianites, Nicholson.) 
Featiier-Feactuee in Igneous Rocks. 
On transverse joints of lava columns: — The transverse joints 
of prismatic columns of lava from Staffa exhibit a surface of fracture 
identical with that in the shales above described, the difference lying 
in the nature of the boundaries of the area of splitting. There is a 
roughly defined central areole, without feather-fracture, from the 
bounds of which off radial lines due to the imbrication of minor 
fracture-planes and the cross-fractures between them. (See Plate 2 , 
fig. 8.) In this case simple contraction is the most probable 
explanation of the rupture. 
On the joints of columnar structure: — That the feather-fracture 
is sometimes due to mere splitting of the rock through cooling and 
contraction is proved by the occurrence of these fracture lines on 
the faces of the joints in the columnar structure of dike rocks. The 
feather-fracture is invariably present on the faces of the rude col¬ 
umns in the diabase dikes of the Somerville quarries in Massachusetts. 
The figures show divergence in different directions, indicating the 
direction in which the fractured surface spread. The system of 
fractures is generally coarser in the middle of the dike than near the 
contact. The feather structure is entirely distinct from the brick- 
work surfaces of fracture which appear, for instance, on the columns 
in O’Rourke’s quarry at Orange, N. J., and also at Titan’s Piazza in 
the Connecticut Valley. The occurrence of the feather-fracture on 
these columnar faces is the effect of the direct splitting of the rock 
through separative movements induced by contraction. In the case 
of vertical dikes, where the cooling must have gone on equably with 
reference to the opposite walls, there seems no reason for appealing 
to torsion as a factor in the production of these fractures. 
