■±-14 Ilobhs — Geological Structure of the 



fault system. Additional observations of value for determin- 

 ing the system of faults as a whole are: 



1. The arrangement of rectilinear formation boundaries in a 

 network of parallel series. 



2. The arrangement of individual faults in intersecting 

 parallel series. 



3. Determination of the joint system and its relation to the 

 fault system. 



I. Zigzag topographic relief composed of straight elements 

 in parallel series. 



5. The drainage system a network of intersecting parallel 

 series. 



With abundant outcrops a fault system may be disclosed 

 upon the areal map by the generally rectilinear or zigzag 

 boundaries of formations. If not, some indication may be 

 afforded by the prevalence of straight lines and sharp bends in 

 the topography, particularly if the lines fall into intersecting 

 parallel series. Such a relief may not be apparent without 

 careful examination of the map, for the reason that the particu- 

 lar combination of directions is not known, and it may yet be 

 quite striking when once discovered. Again, the drainage 

 lines may compose a network which in direction conforms both 

 to that of the topographic lines and that of the formation 

 boundaries. The prominent joints observed at the individual 

 exposures, as was long since shown by Daubree, will, if of the 

 same 'origin as the fault planes, conform in direction to the 

 directions of these planes. 



Application of Methods. 



In applying the above principles to the structural study of 

 the crystalline belt in southwestern New England, advantage 

 has been taken of two important considerations. 



1. The areas of most intricate areal development of the for- 

 mations are key areas for determination of the manner of 

 deformation. — In geological mapping it is almost an unwritten 

 law that geological sequences must be established in areas 

 where formations appear in their larger masses at the surface, 

 it being generally assumed that structural relationships are in 

 such cases the simpler. Those areas, on the other hand, which 

 exhibit a considerable number of formations brought closely 

 together in small masses at the surface are apt to be looked 

 upon with suspicion as areas of local and so-called "minor" 

 faulting, or as containing intercalated beds of unusual types 

 due to purely local conditions of sedimentation. Whichever 

 of these views, be assumed, the areas are likely to receive but 



