600 A TREATISE ON METAMORPHISM. 



less closely, and for the planes to be in parallel sets. In a given case there 

 may be one or more of these parallel sets ; commonly there are as many as 

 three. In such instances the rocks are broken into parallelopiped blocks. 

 In some instances there are as many as six or more parallel sets of ruptures, 

 and by them the rocks are cut into various irregular forms. 



The spacing of the different sets of ruptures is very variable. For 

 instance, if there be three sets of ruptures, the distances between the frac- 

 tures in one set may be several or many times as great as between those of 

 another set. The spacing of all the sets in one case may be much wider 

 than that of any set in another case. The fractures may be very far apart, 

 as Avith widely separated faults or joints, or they may be very close 

 together. In some instances the different sets of ruptures are so close 

 together that it is scarcely possible to find an unruptured parallelopiped a 

 centimeter in diameter. Ruptures of this kind are very well illustrated in 

 the slates and graywackes of the Wausau district of Wisconsin. However, 

 in the belt of cementation the ruptures are not so close as to cut all, or even 

 any considerable proportion, of the mineral particles. Between the amount 

 of rupturing necessary to produce particles a fraction of a centimeter across 

 and that necessary to affect every mineral particle there is a veiy wide gap. 

 In general, near the surface the ruptures are likely to be widely spaced, 

 with considerable differential movement along the fractures. As depth 

 increases, so that the rocks are subject to the weight of the superincumbent 

 material, the ruptures are likely to be closer together. When the depth 

 become so great that the fractures affect the individual mineral particles, 

 the zone of anamorphism, or rock flowage, has been reached. 



The fractures produced in rocks must take place as a result of tensile or 

 compressive stresses." Where the rocks are homogeneous and the fractures 

 are by tensile stresses, they take place normal to the force. Where the 

 fractures are by compressive stresses — and this is probably much more com- 

 mon — the ruptures are inclined to the direction of maximum compressive 

 stress. Ignoring the effect of the stress, and supposing the rocks to be 

 homogeneous, the ruptures would occur in sets at angles of 45° to the 

 maximum stress. However, since the stress itself gives additional strength 



a Becker, G. F., Finite homogeneous strain, flow, and rupture of rocks: Bull. Geol. Soc. America, 

 vol. 4, 1893, pp. 48—19. Van Hise, C. R., Principles of North American pre-Cambrian geology: Six- 

 teenth Ann. Eept. IT. S. Geol. Survey, pt. 1, 1896, pp. 668-678": 



