OBSERVATIONS AND EXPERIMENTS ON JOINT PLANES 65 
dip joints. These joints are strong, especially in the sandy layers, 
where they are conspicuous. As in the strike set, the hade is 
small and uniform. In a single small area the strike of the dip 
joints varies less, perhaps, than the strike of the strike set, but 
from place to place the variation is greater. Farther down the 
creek the average angle with the north is smaller. In upper Fall 
Creek there is little evidence of a second dip set, common elsewhere, 
which makes a small angle with the set just described. In Fig. 7, 
locality 55, is shown the average of some poor joints which may 
belong to this second dip set. 
Besides these, certain joints with much more variable strike 
between 60° W. and W. might be considered as a set. In some 
places where the strike set is weak this set has a development 
which, though less regular than in the strike and dip sets, is quite 
strong and distinguishes the set from the mass of small joints. The 
hades are usually larger and less uniform than in the strike and dip 
sets. These joints are often curved and the smaller ones, espe- 
cially at Forest Home, often show a sigmoid horizontal outcrop 
with the hade varying from one side to the other with the curve. 
The rest of the joints of this area may be classed as weak and 
variable. For thirty degrees east of north the variable joints are 
more common and may be due to the tendency for a major set to 
form in that direction, but in the upper Fall Creek gorge they do 
not form a recognizable set. Variable joints a foot or two in length 
strike toward every point of the compass. Their hade is usually 
high, from 30° to 60°, and as a rule both the strike and hade vary 
over even the small extent of these joints. Enough readings were 
made on these to indicate their general character. .They are 
common, but unimportant in comparing the variation of the joints 
with the folds. 
The study in Fall Creek showed that joints of all directions are 
present but that those of different directions vary greatly in charac- 
ter. There are two sets in which the individual joints are strong 
and, what is more important, each joint is nearly the same through- 
out its extent and the joints of each set are nearly constant for one 
locality. It was apparent that these could be used in comparing 
different areas but readings on the variable joints would be of little 
