6l4 STUDIES FOR STUDENTS 



are not joints of other origins, but merely that the master joints, 

 which run in different directions approximately parallel to the 

 strike and dip, may be thus explained, and these are the joints 

 which are the most useful in determining the structural relations 

 of different series. 



ZONE AFFECTED BY JOINTS. 



Joints, implying as they do openings in the rocks, are neces- 

 sarily confined to the outer zone of fracture and the middle zone 

 of fracture and flowage. In the first zone they are of more 

 importance and probably more regular than in the second. In 

 the deeper zone of rock-welding no joints can develop. In 

 rocks once buried to this depth, which subsequently reached the 

 surface by erosion, joints may be formed ; for in approaching 

 the surface they passed from the zone of flowage, through the 

 zone of fracture and flowage, into the zone of fracture. 



RELATIONS OF JOINTS TO STRATIGRAPHY. 



If there is a greater number of sets of joints in an inferior 

 formation or formations than in a superior formation or forma- 

 tions, the two divisions being of such lithological character as to 

 be equally likely to take on jointing, this argues that there may 

 be discordance between the two sets, for it is probable that the 

 lower set of formations, which has the more complicated joint- 

 ing, was subjected to orogenic movements which produced a part 

 of these fractures, before the upper series was deposited. Com- 

 stock has applied this as the determining criterion in separating 

 three supposed series of rocks in the Llano district of Texas. 

 The lower series of formations is said to have three sets of pro- 

 nounced joints running in definite directions, while the middle 

 series has only two sets of joints running in definite directions, 

 these two being common with two of the three in the lower series, 

 and the upper series has a single set of joints running in a defi- 

 nite direction, this system being common to both the inferior 

 series. Comstock's inference is that the system of joints in the 

 lowest series not found in the upper two series was produced 



