208 Van Hise and Hoshins — Pre- Cambrian Geology. 



produce deformation in two directions at right angles to each 

 other, this results in the production of complex folds. The folds 

 produced by the major thrust may be called major or longitu- 

 dinal folds, and the folds produced by the minor thrust, minor 

 or transverse folds. The methods of determining that a dis- 

 trict is one of complex folding and the character of the observa- 

 tions which should be made in such a district are fully discussed. 

 In the production of simple, composite, or complex folds the 

 movements may have been continuous or discontinuous. 



Cleavage and Fissility. — The property of cleavage in rocks 

 is defined as a capacity present in some rocks to break in cer- 

 tain directions more easily than in others. Fissility is defined 

 as a structure in some rocks by virtue of which in a state of 

 nature they are already separated into parallel laminae. The 

 term fissility thus complements cleavage, and the two are 

 included under cleavage as ordinarily defined. It is held that 

 cleavage is a deep-seated phenomenon of the zone of flowage, 

 and that fissility is a more superficial phenomenon of the zone 

 of fracture. 



From a discussion of the phenomena it is concluded that 

 rock cleavage as above defined is due to the arrangement of the 

 mineral particles with their longer diameters or readiest cleav- 

 age, or both, in a common direction, and that this arrangement 

 is caused, first and most important, by parallel development of 

 new minerals ; second, by the fiattening and parallel rotation 

 of old and new mineral particles ; and third, and of least 

 importance, by the rotation into approximately parallel posi- 

 tions of random original particles. From the fact that fre- 

 quently cleavage is everywhere parallel to intrusive batholites, 

 from the fact that fiattened rock particles are in the plane of 

 cleavage, and from the frequent transverse relations of the 

 structure and the bedding, it is concluded that cleavage devel- 

 ops in a plane normal to the greatest pressure. In homogene- 

 ous rocks the structure may vary from perpendicular to parallel 

 ' to the bedding. 



Fissility, being due to rupture, is produced along shearing 

 planes, and is, therefore, inclined to the direction of greatest 

 pressure. Cleavage and fissility intersecting bedding are called 

 cross cleavage and cross fissility, and those parallel to the bed- 

 ding are called parallel cleavage and parallel fissility. 



In heterogeneous rocks the readjustment between the beds 

 may rotate the cleavage from its ordinary position, but, as 

 shown by Prof. Hoskins, at any moment the cleavage is devel- 

 oping in the direction of greatest shortening, or at right angles 

 to the greatest pressure. The result of the rotation of cleavage 

 is to bring it more nearly in accordance with bedding. As the 

 readjustment and consequent rotation is greater in a weaker 



