REVIEWS 425 



of dip of the minor or cross folds. The average strike is therefore 

 determined." 



3. " Determine the direction and pitch of the axes of the minor folds. 

 The first is the direction of dip, and the second the amount of dip of 

 the major folds. The average strike is therefore also determined." 



Van Hise correctly observes that of these three observations the 

 first is the only one ordinarily taken, and it is the one of least impor- 

 tance in regions of close complex folding. It becomes obvious in 

 considering his practical suggestions that a thorough knowledge of 

 the physiography of the region and the relation of relief to structure 

 is quite as important for the solution of complex folds as is a knowl- 

 edge of the rocks. 



Concluding the section relating to folding, Van Hise considers the 

 value of differential folding as an evidence of unconformity, when not 

 sustained by other phenomena. A discordance in the flexures of two 

 adjacent formations is only to be absolutely relied upon as evidence 

 of unconformity when the structural discordance is so marked that the 

 earlier series exhibits a relatively complex structure as compared with 

 that of the later series. 



In Section III Van Hise considers cleavage. He grasps hypotheses 

 of the origin of cleavage advanced by other geologists from Phillips 

 to Becker, discriminates between them, amplifies their basis of fact by 

 his own extensive observations of the crystalline schists, and states 

 those conclusions which he finds valid. He defines cleavage in a 

 strict sense "as a capacity present in some rocks to break in certain 

 directions more easily than in others." This use of the term is that 

 in which it is applied to minerals. He distinguishes fissility as a 

 phenomenon secondary to cleavage, fissility being " a structure in some 

 rocks by virtue of which they are already separated into parallel laminae 

 in a state of nature." 



Citing microscopical study of cleavable slates and schists, Van Hise 

 concludes (page 635) that : 



" Rock cleavage is due to the arrangement of the mineral particles 

 with their longer diameters or readiest cleavage, or both, in a common 

 direction, and that this arrangement is caused, first, and most impor- 

 tant, by parallel development of new minerals ; second, by the flatten- 

 ing' and parallel rotation of old and new mineral particles; and third, 

 and of least importance, by the rotation into approximately parallel 

 positions of random original particles." 



