MAPPING OF THE CRYSTALLINE SCHISTS 785 



Van Hise has shown that the crests of folds are lines of 

 special importance by reason of the fact that the order of super- 

 position of beds is there almost sure to be the normal one, and 

 the additional fact that crushing being there at a minimum, con- 

 glomeratic and other original structures are most likely to be 

 preserved. 



Form of minor folds. — As might be inferred from the last 

 section, the form of the minor fold or the plication is full of 

 meaning respecting the characteristics of the larger folds of the 

 region, and it should, therefore, be carefully studied and its 

 peculiarities sketched or recorded. 



Secondary foliation and its relation to bedding. — Foliation is one 

 of the commonest of the structures in the crystalline schists. 

 It is now well known that in the recent past the strike and dip 

 of foliation have rather generally been measured as those of 

 bedding, so that in most papers, written more than a decade 

 since, the secondary nature of foliation was not recognized, 

 and recorded observations of dip and strike as frequently refer 

 to secondary foliation as to planes of sedimentation. Even with 

 the larger knowledge of the present, the differentiation of the 

 two structures is difficult and often impossible. Two, three, or 

 even more sets of planes of separation, with widely different 

 directions, may all be easily made out in the same hand speci- 

 men of clastic rock, and yet no one of them correspond in direc- 

 tion with the bedding plane. 



A difference in composition characterizing alternate parallel 

 bands of considerable thickness is undoubtedly the best criterion 

 for determining the plane of sedimentation. 1 A striking instance 

 of secondary banding is represented in Plate I, where the 

 bedding plane is outlined by the crumpled lenses of quartz and 

 by even more beautiful puckered bandings not discernible in 

 the view. The secondary straight banding is parallel to the 

 foliation. (Plate XIV of the work last cited is from near this 

 locality.) Such a structure, or one resembling it, may, however, 

 be produced by the mashing of a coarse granite or conglomer- 



1 See, however, Hobbs, "Secondary Banding in Gneiss," Bull. Geol. Soc. Am., 

 Vol. Ill, pp. 460-464, PI. XIV. 



