180 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
based. If some combination of fractured surfaces analogous to 
b-planes and c-fractures, or to the curved surfaces of fractures which 
represent both, is characteristic of the margins of great fractures, 
or arise as independent fractures, we ought to find some expression 
of this system of fissures on portions of the earth’s crust, as where 
many large dikes show ‘‘bridges” crossing from wall to wall. It 
will be noted that the outcrop of these marginal fractures on a plane 
perpendicular to the fissure gives rise to several types of surface 
fracture, each of which at a depth may merge into a greater and 
uniform plane of fracture. 
The comparison with great fissures which the geologist naturally 
first seeks to make is found in lines of volcanoes, forming: ranges 
along the coast of a continent or over the sea floor. 
So long as intermediate fractures ranging in size between the joints 
and fractures of quarry openings and great fissure lines marked out 
by volcanoes are not surveyed, the argument based upon a law of 
fractures observed in joints will be held weak. It is favorable to the 
argument that the transverse bridges between fractures oblique to the 
main line of fissure are characteristic of the crevasses of glaciers, 
and such fractures have been actually observed in dikes about 
Boston, Mass. It does not follow, however, that these fractures, 
are the mere border phenomena of a more deep-lying continuous 
surface of rupture. Even in the case of small joints in the hand 
specimen it may be observed that the whole structure is that of 
imbricated fractures forming oblique plates of rock which have in 
turn snapped across. If, too, it be objected that the great volcanic 
fissures are in their smaller fractures bordered by oblique fractures, 
it may be stated that the oblique planes in joints are composed 
of yet smaller oblique planes with cross-fractures and so on down to 
a fineness of subdivision that has not yet been investigated. The 
system of fractures in rocks is very complex. 
The elder Dana (’47) early called attention to the occurrence of 
short parallel systems of fracture and uplift in ranges inclined to the 
main course. His diagrammatic illustration is identical with that 
in Plate 1, fig. 6, based on the fringe of joints. The great trends 
are frequently curved, and so are some joints. 
“To produce such result,” he states, “must there not be two dis¬ 
tinct causes, a force to rupture, and a structure to determine the 
direction of the lines? Were there no structure, the force should 
have produced a fissure in the general direction of the dotted line 
