MAPPING OF THE CRYSTALLINE SCHISTS 889 



It should be by a consideration of all the above outlined 

 methods that the presence or absence of a fault system should 

 be determined for any given area. 



METHOD OF MAPPING A CRYSTALLINE AREA DEFORMED BY A 

 SYSTEM OF FOLDS IN CONJUNCTION WITH A SYSTEM OF 

 FAULTS. 



The discovery that the beds within a region of crystalline 

 schists have assumed their present attitudes in consequence of 

 deformation of the area by a system of folds in conjunction with 

 one of normal faults, greatly increases, it must be admitted, the 

 difficulties in the way of accurately setting forth the geology 

 upon maps. The question may well be asked whether even with 

 patience and industry the areal distribution of formations can be 

 adequately represented. Where outcrops are abundant and well 

 distributed, and where formations are sharply differentiated 

 petrographically, the difficulties can be overcome ; but where 

 gneiss, schist, and quartzite types by reason of gradational mem- 

 bers are with difficulty distinguished, even under favorable con- 

 ditions, their appearance in a fault mosaic will in many cases 

 defy the most skillful worker to unravel. 



To fix the order of succession of formations, it will be neces- 

 sary, not only to exclude the possibilty of an overturn (as by 

 finding the arch of a cross fold), but actually to follow one 

 formation beneath the other. 



In other ways the problem of mapping will be simplified. 

 The appearance of formations, first in one areal succession, and 

 again in a different succession not to be accounted for by overturn- 

 ing, will find an adequate explanation. Upon the theory that 

 folding alone has accomplished the deformation of an area of 

 crystalline rocks, geological sections have been strained literally 

 and figuratively beyond the point of rupture. The mechanics of 

 folding is now pretty well understood, and the appearance in 

 geological sections of synclines which have one limb several 

 times the thickness of the other (a not uncommon means of adjust- 

 ing to theory) finds, it is believed, as little warrant in mechanics 

 as it does in observation. 



