OBSERVATIONS AND EXPERIMENTS ON JOINT PLANES 63 
the center of a domed anticline looking from the south whence the 
active force came. Apparently the strike of the faults is about 
parallel to the strike of the rocks on the southern limb. If the 
pitching of a fold is due not to a variation in the active force caus- 
ing folding but to a variation in the rigidity of the rocks, the mole- 
cular return forces in the soft rock near the center of the dome 
would not be so great as in the harder rock near the saddles and 
movement toward the dome might be expected. The exact analy- 
sis of the faulting is a problem in shearing closely connected with 
the shear theory of jointing. 
OBSERVATIONAL WORK 
JOINT PLANES 
In measuring the joints a compass with a four-inch needle and 
open sights was used. The deflection of the needle from true north 
was taken roughly as seven degrees west. No attempt was made 
to read to less than a degree. The accuracy of the readings 
depended upon the character and exposure of each joint. The 
observations on most of the master joints were accurate to one or 
two degrees, but for the variable minor joints of large hade the 
error might be from five to ten degrees. The hade was measured 
with a six-inch protractor to which a lead was attached by a thread. 
Wherever possible the measurements of hade were made from such 
a distance that the edge of the protractor covered nearly the whole 
height of the exposure in order to obtain a good average. The 
readings were usually made to one degree. The master joint read- 
ings were mostly correct to one degree but for minor joints the 
error might be several degrees. 
FALL CREEK JOINT PLANES 
In the upper part of the Fall Creek gorge all accessible joint 
planes except the smallest and most variable were measured. 
Fig. 4 shows the orientation of these joints. The readings are 
tabulated by the method used by Professor Tarr for the joints of 
Cape Ann.!. The strikes are divided into groups of three degrees, 
t Ninth Ann. Rep. U.S. Geol. Surv., 1887-88, pp. 583-88. 
