468 SRODMES MOK SIO DENS: 
the case of parallel fissility the movement may be compared to 
the differential movements of a ream of paper which under load 
Pit. 5 
Fic. 6 
Fics. 5 and 6.— Theoretical deformation of laminated rock by uniform differential 
movements parallel to a previous structure. 
is pushed unequally forward from one end, the differential move- 
ment between any two successive sheets being thesame. While 
the bottom sheet may be shoved one-tenth of an inch, the top 
sheet may be shoved 10 inches (Figs. 5 and 6). Between the 
bottom and the top the amount of forward movement would 
vary regularly from one-tenth of an inch to 10 inches. While 
the differential movement between any twosheets is the same, 
each sheet gains all the forward movement of all the sheets 
below. 
Where fissility develops, either as an original or secondary 
structure, rubbing usually occurs between the mineral particles 
