786 STUDIES FOR STUDENTS 



ate, but the characteristic structures of these rocks are usually 

 not obliterated unless the bands are reduced to a thickness of 

 a quarter of an inch or less. Again injection of granitic mate- 

 rial along the planes of foliation of a rock may produce an 

 alternation of bands of different composition. Such a structure 

 is, however, apt to be recognized by an experienced observer, 

 and unless subsequently mashed the bands are not sufficiently 

 uniform in thickness to simulate bedding. 



Of all the crystalline rock types of clastic origin, quartzite 

 offers the greatest difficulties in the search for the bedding plane. 

 In fact, it is generally impossible to determine it except by con- 

 tacts with other formations. 



Other things being equal, the more crenulated or wavy a 

 structure, the greater the probability that it is original. Also 

 the more its general direction diverges from an undisturbed 

 plane of foliation the more likelihood is there that it is earlier 

 and original. When the bedding plane cannot be made out with 

 certainty at a particular locality it may sometimes be inferred 

 from a study of the form of the minor folds in their relation 

 to foliation at a neighboring locality. Owing to the fact that 

 mashing is at a minimum in the arches of folds, careful' search in 

 an area of closely compressed folding will often reveal at regular 

 intervals the beautifully crenulated arches with foliation planes 

 bisecting them, although between such localities the foliation 

 planes only can be found. Such a method may even be applied 

 to a slate. The green and purple slates of the Taconic range of 

 mountains dip to the eastward persistently and quite uniformly. 

 The writer was able to find at fairly regular intervals across the 

 strike of this foliation the acute crests of easterly dipping 

 anticlines preserved in lenses of infiltrated silica, thus showing 

 that a series of moderate thickness has, by buckling and com- 

 pression, brought about perfect foliation parallel to the axial 

 planes of the folds, and thus given the impression of vast thick- 

 ness. 



Van Hise, in his treatment of the mechanics of flexuring — 

 confirmed by field observation — has shown that in moder- 

 ately compressed folds the foliation planes of the opposite 



