WOODWORTH: FRACTURE SYSTEM OF JOINTS. 
177 
The coarse shale structure is exemplified in the upper beds of the 
Mystic River section in Somerville, Mass. Here the shale structure 
forms open lines along the faces of joint-planes, while the banding 
of the pelites is uniformly sealed, so that the rock splits up into 
llattish pieces along these secondary planes of division. When 
examined on their llattish surfaces, these plates are found to be 
marked by radiating lines, proceeding outward toward the margin 
of fracture from a subcentra] areole, which is from an eighth to a 
quarter of an inch in diameter, and in which the appearance of 
radiation is wanting. The surface of the rock within these small 
areoles is usually smoother than elsewhere. Under the lens, the 
texture is neither finer nor coarser than in the surrounding rock. 
(Plate 5 , fig. 6.) These structures correspond to the discoidal joints 
found in the joint-rock of the lower zone of pelites. The radiating 
lines, so marked to the unaided eye, appear under the lens of moderate 
power as rounded ridges and troughs with minute hackly surfaces, 
with here and there planes of fracture cropping out at very low 
angles and having their edges parallel with the radiate structure. I 
believe that I have observed that these mostly dip in one way 
in succession round the central areole, as they do in the discoid 
joints. The margins of the figure of fracture terminate rather 
indefinitely, by merging into the plane of other figures to the right 
and left. Frequently at the margin of two fractures there is a 
Ioav ridge with a sharp edge, with a corresponding trough in demi- 
relief on the contiguous piece of rock. The concentric lines of the 
conclioidal structure are rarely if ever developed, as they sometimes 
are in the discoidal joints. In some specimens, there is a marked 
tendency to form an axis along one of the radii from which the 
splitting figures diverge with the curl peculiar to the feather-fracture 
of typical joints. Slickensides have not been observed in any part 
of the fractured surfaces. The phenomena are clearly due to simple 
separation of the rock. 
Comparison with Taonurus , etc.: — At first sight the radial 
shale fracture recalls Spyrophyton cauda-galli , Taonurus, etc., com¬ 
monly believed to be of organic origin. Nathorst has stated (’82, 
p. 90) as his opinion, based upon experiments, that Spiropliyton is 
of mechanical origin, and he ascribes the structure to a process con¬ 
temporary with the deposition of the sediments: — 
“ C’cstpourquoi,” says he, “ je dois dire que, suivant mon opinion, le 
Spiropliyton est du a un tourbillonnement produit dans l’eau par une 
