598 STUDIES FOR STUDENTS 



mountains may be carved. As each stratum grinds over the one 

 below it the former presses against the latter with all the weight 

 of the superincumbent material. Under such circumstances it is 

 no wonder that a coarse-grained, massive granite may be trans- 

 formed into an evenly laminated schist. In the zone of fracture 

 the schist, developed in the zone of flow, may become fissile, and 

 the slickensided, wavy folia may be thinner than paper. It is 

 by folding combined with differential movement that the abnor- 

 mal folds described on a previous page are produced. 



The cleavage or fissility so frequently present upon the flanks 

 of anticlinal core-rocks in great mountain ranges may be 

 explained by similar movements. In the section on the analysis 

 of folds it has been shown that much readjustment must occur 

 upon the limbs of folds. The flanks of an anticlinal mountain 

 core are such limbs, and hence the development of cleavage or fis- 

 sility parallel to the central massif. In passing toward the center 

 of the core we approach nearer the crown of the anticline, and 

 penetrate to a greater depth ; hence less readjustment is necessary, 

 and therefore the secondary structures are less prominent. 



RELATIONS OF CLEAVAGE AND FISSILITY TO THICKNESS OF STRATA. 



Without reference to the origin of secondary structures, or 

 any evidence upon this point, bedding and secondary structures 

 are often spoken of as corresponding. Even reputable text- 

 books make such statements. This confusion is most unfortu- 

 nate for two reasons. (i) It often leads to great overestimates 

 of the thickness of strata, the real thickness of the beds being 

 supposed to be the apparent thickness as observed across the 

 secondary structure, where, as shown by the foregoing analysis, 

 the same bed may be repeated many times. (2) The mistake 

 is likely to give erroneous ideas of structure. If the primary and 

 secondary structures are thought to correspond, the whole 

 breadth of a slate or schist may be regarded as a bed of enor- 

 mous thickness, and this will lead to the preparation of sections 

 in which the mass is represented as extending to a great depth, 

 where it may be comparatively superficial. (Fig. 12.) 



