64 PEARL SHELDON 
each beginning at the west so that the outer rays are N. 90° W.— 
N. 88° W. and N. 87° EN. 89° E. The figures in the margin give 
the number of readings to each group of compass directions. 
Fig. 4 shows that in direction the joints fall conspicuously into 
groups or sets. One set whose strike is usually between N. 70° E. 
and N. 80° E. is strong and nearly constant in direction and hade. 
Although the extreme readings of strike in this set vary by twelve 
or thirteen degrees the majority fall within four or five degrees. 
The hade is also nearly uniform. At locality 57, one hundred forty- 
six joints belonging to this set were measured. ‘The total range of 
Fic. 4.—Tabulation of the strike of the Fall Creek joint planes 
hade was from 62° S. to 9° N., but only two of the entire number 
showed an inclination to the south and most of those to the north 
fell within a range of a very few degrees. This set is well developed 
at Forest Home, locality 57, but not elsewhere in Fall Creek. 
Localities 55, 56, and 58 afford only a few readings and those not 
very good. This set is best developed in the shale beds, and the 
variation: with the character of the rock may account for its pres- 
ence at Forest Home, where shale beds are common, and its poor 
development in the more sandy layers above and below. The 
joints of this set strike nearly parallel to the strike of the rocks and 
form the set known as the strike joints. 
Nearly at right angles is another set most of whose readings are 
between N. 10° W. and N. 15° W. ‘They represent the set called 
