OBSERVATIONS AND EXPERIMENTS ON JOINT PLANES 67 
average. A width equal to half a mile on the scale represents one 
hundred readings. Where the joints in any area vary in direction 
more than is usual in this set the average direction is given in light 
figures. Where the variation is less than usual the figures are 
heavy. — It will be seen from Fig. 6 that the average directions are 
nearly parallel to the axes of the folds. It is noticeable that for the 
four localities along Salmon Creek, where this set is well developed, 
the average directions do not vary half a degree, though the indi- 
vidual readings vary by several degrees. There is a certain actual 
variation from place to place not due to the thoroughness with 
which observations were made. For example, just south of Crow- 
bar Point and also on the opposite side of the lake the average 
angle is unusually low even though the number of readings is 
sufficient to give a reliable average. The cause of such deviations 
probably lies in the local variations of the forces producing the folds 
and joints. Near the Shurger Point anticline, which rises to the 
east so that the strike of the rocks is not parallel to the axis, the 
average strike of the joints does not turn so that it is parallel’to the 
strike of the rocks, but in the opposite direction so that the strike 
of those south of the axis points slightly in toward the center of the 
domed anticline. Just how the strike joints vary with the pitching 
of folds cannot be determined from this one region. Further study 
of well-developed strike joints near pitching folds is necessary to 
warrant conclusions. In the southern part of the area studied the 
strike set is too poor to give reliable evidence. 
Effect of the rock character——The variation between the two 
averages near the mouth of Taughannock Creek is due to the 
character of the rock. The upper readings were in the Tully lime- 
stone, the lower in the Hamilton shales directly beneath the lime- 
stone. Few readings were made in the Tully elsewhere but a 
similar variation from the strike in the shales was noticed in other 
places. The hardness of the rock has a decided effect on the strike 
joints. They seem best developed in homogeneous shales, espe- 
cially the Hamilton beds. This is partly indicated by the abun- 
dance of readings on this set in the northern part of the map. South 
of the lake the rocks are hard Portage sandstones and shales, and 
the meagerness of readings, though partly due to poorer rock out- 
