156 Transactions of the Academy of Science of St. Louis 
Faulting consists of movements parallel to the walls of a 
fracture and is therefore due to shearing. Thrust faulting is 
due to compression and results in crustal shortening while nor- 
mal faulting is due to tension and results in crustal elongation. 
Both normal and thrust faults are parallel to the shearing planes 
of the strain ellipsoid. In many regions of faulting there are 
vertical faults at right angles to the thrust faults which are 
parallel to the axes of least and greatest stress as set up for 
the thrust faults. The vertical faults are the result of horizon- 
tal movements and involve neither crustal lengthening or short- 
ening. The term flaw is coming into general use for this type 
of faulting (35). 
An explanation of flaws lies in the production of secondary 
stresses by the movement due to primary stresses. Differential 
movement along the zone of thrust faulting caused by varying 
weakness in the rocks acted upon, will set up horizontal stresses. 
The vertical faults will form along the shearing planes of strain 
ellipsoids whose intermediate axes are vertical as the axes of 
least strain must be horizontal in order to relieve the stress. 
These relations may be seen in Plate [Xp which shows the sec- 
ondary ellipsoids as smaller than the ellipsoids produced by the 
major stress. 
After the secondary, vertical faults have formed, relief of 
pressure may allow the formation of normal faults back of the 
thrust faults, leaving the space between faults as an upthrown 
block. The formation of upthrown blocks by longitudinal and 
transverse faulting has been advocated by Stoces and White (41) 
who believe, however, that the longitudinal faults on both sides 
of the block are thrust faults. They also state that the trans- 
verse faults form only when resistance to folding or thrust 
faulting becomes too great to allow relief by either of those 
types of movement. 
Joints are fractures along which there has been no move- 
ment parallel to the walls. They are formed by tension normal 
to the joint planes which are oriented parallel to the axes of 
. greatest and intermediate stress of the strain ellipsoid. Two 
