794 PRINCIPLES OF STRATIGRAPHY 



actual cleavage or splitting is subsequently developed by the frost, 

 or other atmospheric agent, or by the hand of man. 



Coarse-grained rocks are seldom affected by cleavage. Lime- 

 stones likewise offer great resistance to compression, and are not 

 generally cleaved. Thus cleavage may often be developed in a 

 stratum of argillaceous lutytes, while adjoining arenytes or calcare- 

 ous beds will be unaffected. This sometimes gives rise to the ap- 

 pearance of an unconformity. 



Cleaved strata commonly have their original bedding structure 

 obliterated. Only in exceptional cases are the bedding planes pre- 

 served, when the strata are differently colored or when a change 

 in texture occurs. In such cases ribbon slates or banded slates are 

 produced. In cleaved fossiliferous strata the fossils may sometimes 

 be detected on the weathered surfaces of the bedding plane, the 

 position of which they indicate. A very general distortion of the 

 fossils accompanies the formation of cleavage, so that in many cases 

 the remains are no longer recognizable. 



Unless the relation of cleavage to the bedding is detected, an er- 

 roneous conception of the structure of a country is obtained. Strata 

 which are strongly cleaved generally appear to be vastly thicker 

 than they really are, and unconformities are sometimes considered 

 to exist between strata where in reality the bedding planes are per- 

 fectly concordant. If, however, a formation with slaty cleavage is 

 overlain by one without such structure, although of a composition 

 which would permit its development as readily as would the under- 

 lying rock, a discordance of relation is indicated, though in the ab- 

 sence of other evidence this is not fully demonstrated. (Van Hise- 

 23:7^(5.) 



15. Fissility is the structure found in some rocks, "by virtue of 

 which they are already separated into parallel laminae in a state of 

 nature." It thus differs from cleavage, where this separation is 

 only potential. Fissility belongs in the zone of fracture, while 

 cleavage belongs in that of flowage. Both occur and grade into 

 each other in the zone of combined fracture and flowage. 



16. ScJiistosity. This structure is the result of intense meta- 

 morphism under pressure, and is characterized by the development 

 of planes of cleavage due to the presence of large, cleavable par- 

 ticles. It is essentially comparable to slaty cleavage, except that 

 metamorphism has gone farther and the rock has become crystal- 

 line. Schistosity may be developed in rocks of many kinds, both 

 clastic and igneous. Foliation is another term applied to these 

 rocks. 



The structure is essentially due to recrystallization of the rock 



