MAPPING OF THE CRYSTALLINE SCHISTS 869 



spaced with an approach to uniformity. As regards the larger 

 displacements of the area, it was found that they seldom occur 

 wholly along one plane, but are distributed over a zone of 

 parallel and near-lying planes — the throw is distributed. 



The faults of the less common series run along the diagonals 

 of the larger composite blocks, and are believed to owe their 

 formation to the downthrow of these blocks, and not to the 

 original relief of the area from compression. 



CRITERIA FOR RECOGNIZING A SYSTEM OF FOLDS. 



While not necessarily present in all areas of disturbed rocks, 

 it is fair to assume that in all areas of the crystalline schists in 

 which bedded rocks are found, folds will be recognizable. The 

 possibility of unraveling their relations to one another within a 

 system will depend upon the intricacy of the structure, the fre- 

 quency of the exposures, the freshness of the rock, the perfection 

 of a combined joint and fault system, etc. 



In general it may be said that flow structures and those pro- 

 duced by fracture are in contrast, the former exhibiting warped 

 (curving) surfaces, and the latter plane surfaces. The courses 

 of the fold structures across the map are, therefore, curved lines, 

 whereas the latter course over the map in straight lines, or in 

 zigzags made up of rectilinear elements. The courses of fault 

 structures upon the map are little disturbed by the relief of the 

 topography in proportion to their inclination — hade, being abso- 

 lutely unaffected by topography when the hade is 90 . The 

 nature of known formation boundaries upon the map, and that of 

 other structures is thus one of the most important considerations 

 in the determination of the kind of deformation to which the 

 area has been subjected. When alternations of pitch of folds are 

 not present, folded rocks present also rectilinear boundaries, but 

 this consideration loses importance in most areas of intricate 

 geological structure, because evidences of pitch can generally be 

 made out. Change of pitch has been described as characteristic 

 of few areas, because, as already explained, it has until compara- 

 tively recently seldom been looked for. 



Other indications of the presence of flexures within a given 



