OBSERVATIONS AND EXPERIMENTS ON JOINT PLANES 75 
that the dip joints do not belong to a single set. They fall into two 
groups, with sometimes one and sometimes the other more promi- 
nent. Practically all the strong regular joints of small hade which 
do not belong to the strike set lie in the general direction of the dip 
of the rocks, but a curve drawn between the compass directions 
and the number of joints striking in each direction for a locality 
will usually show a tendency toward two maxima. That is, the 
dip joints form two groups with the average of one group nearly 
perpendicular to the axes of the folds, somewhat west of north, and 
the average of the other set farther east. There may be readings 
continuously between the two averages, or there may be a gap with 
no readings in the middle. There is a decided bunching of strikes 
toward the extremes rather than a larger number of readings near 
the average value of all the dip joints. 
The strike joints show no such tendency. With them a curve 
between strikes and number of readings has a decided maximum 
near the median value with the number of readings decreasing 
rapidly toward the extremes. In one or two places strike joints 
were seen crossing each other, that is, fairly strong joints with 
directions near the extreme range for the strike set would occur 
at the same place and consequently intersect, but this is rare and 
the strike joints clearly form a single set. The dip joints often 
cross each other. In many areas only one dip set occurs, as is 
shown in Fig. 7. In such places it is usually found that the average 
of all the dip joints is about the same as one of the averages obtained 
by dividing the readings where both sets occur, thus justifying the 
division in the latter case. In better cases both dip sets occur 
together strongly and nearly equally developed, the two sets mak- 
ing so large an angle with so few intermediate joints that one would 
not consider averaging them together as a single set. 
Around the city of Ithaca the more westerly of the dip sets is 
dominant. Northward the more easterly set is often the stronger. 
At Lick Brook the two sets are about equal and do not intergrade. 
With the set north of west they cut the rocks into conspicuous tri- 
angles instead of the more common parallelograms where only one 
dip set is strong. Southward from Esty Glen the dip sets are about 
equal, the angle between them is comparatively large, and each 
