Woodworth.— Joints. 
PLATE 5. 
(Photographs by P. P. Sharpies.) 
Fig. 1. Curved end of elliptical joint-plane dying out between upper and 
lower surfaces of stratum, showing greatly enlarged terminal 
border-plane of fringe, with feather-fracture diverging to the 
left. The b-planes are right-handed above and below the joint- 
plane, which is here very narrow. Mystic River quarry. Length, 
180 mm. 
Fig. 2. Joint plane with fringe, here also right-handed but with c-fracture sur¬ 
faces facing outward. Mystic River quarry. Length, 145 mm. 
Fig. 8. Spall of “slate” from Mystic River quarry, showing bulb of per¬ 
cussion below, radiating percussion lines, and conchoidal fracture. 
Length 110 mm. 
o 
Fig. 4. Shaly argillaceous rock from drift at Winthrop Head, Mass., show¬ 
ing group of small discoid joints, parallel to each other but on 
slightly different levels. Diameter of discs, 15 to 25.mm. Length 
of specimen, 115 mm. 
Fig. 5. Discoidal joint, showing b-planes, and c-fractures, with conchoidal 
rim. Old quarry, Tufts College Hill, Somerville, Mass. Length, 
140 mm. 
Fig. 6. Radial shale fractures forming lenticular pieces of rock. Compare 
with fracture of cro§s-joints in columnar lavas (Plate 2, fig. 8). 
Mystic River quarries, Somerville, Mass. Length, 100 mm. 
