Woodworth.— Joints. 
PLATE 2. 
Fig. 1. End view of joint-block, showing joint-plane dying out in rim of con- 
clioidal fracture, aa, joint-plane; bb, rim of fracture. 
Fig. 2. Cross-section of joint-plane, showing inclined border-plane in fringe. 
A, joint-plane, 6, border-plane. 
Fig. 3. Cross section of joint-plane, showing fringe offset by shoulder with 
conclioidal or hackly fracture. See Plate 4, fig. 4. Mystic River 
quarries, a, joint-plane, b, border-plane, d, shoulder. 
Fig. 4. Ground-plan of outcrop of fractures in fringe of joint, giving rise to 
concave faces oh joint-block dd on the right of the open space, and to 
convex faces and b-planes in the joint-block on the left of the space. 
bb, border-planes ; cc, cross-fractures running from the end of one 
border-plane to the middle of the next (curvedlateral cross-fractures). 
Mystic River quarries, and one specimen from Maine, collected by 
N. S. Slialer. See p. 171. 
Fig. 5. Cross-section of discoid joint, showing opposite curving of rim of con- 
choidal fracture at end of one of the diameters of the plane, a, joint- 
plane ; d, conclioidal fracture surface. 
Fig. 6. Cross-section of banded shales, with radial fractures. The lenticular 
lines exhibit cross-section of the structures produced by fractures 
shown on Plate 5, fig. 6. Somerville, Mass. 
Fig. 7. Plan of discoid joint, bb, imbricating planes, dipping largely in one 
direction around the centre, cc, cross-fractures, dd, rim of con- 
choidal fracture. See Plate 5, figs. 4, 5. 
Fig. 8. Diagram of fracture system on transverse joint in prismatic basalt; a, 
central areole. Island of Staffa. 
Fig. i). Diagram showing plan of fissures resulting from b-planes with curved 
lateral cross-fractures. The straight line corresponds to the position 
of the joint-planes in joints, to the general trend of the system in 
great fissures, or in small ones where a single large plane is not 
developed. See p. 171, 181. 
Fig. 10. Block showing fracture composed of a series of curves due to oblique 
b-planes with curved lateral cross-fractures. See p. 181. 
Fig. 11. Diagram of two sets of rectangular joints, with parallel b-planes, 
showing joints developed at angles of 45 degrees to preexisting struc¬ 
ture. Seep. 181. 
Fig. 12. Ideal distribution of volcanoes in couplets along a double line, 
dependent upon the development of oblique subsidiary planes of 
fracture with straight medial cross-fractures. The black dots indi¬ 
cate the points of greatest weakness and favorable sites for volcanic 
extravasation. See p. 182. 
