Southwestern New England Region. 443 



Methods of Investigation. 



Convinced, then, of the presence of a system of faults within 

 the crystalline belt, the writer's next problem was to determine, 

 if possible, its relation to the system of joints, the direction of 

 its component series of parallel faults, and the part which it 

 has played in the deformation of the area. The difficulties in 

 the way of reading at once the composite structure of folding 

 and faulting in rocks so nearly of one petrographical type as 

 are the gneisses of the region, seemed, in the absence of estab- 

 lished methods, to be insuperable, and attention was therefore 

 first directed to those parts of the area in which limestone 

 occurred. In the search for evidence of the system of faults 

 within these areas the most hopeful factor was that rectilinear 

 surface elements are characteristic of joint and fault structures, 

 which are in contrast with the curves characteristic of folding. 

 To this was added the knowledge derived from study of the 

 Newark areas that the post-Newark system of faults, at -least, 

 had not been affected by subsequent folding. 



It may be stated that based upon this and other known 

 peculiarities of faults, methods more or less decisive have been 

 developed which have led to the determination of elements in 

 the fault system of the region. These methods have been 

 elsewhere described* and can be but briefly referred to here. 



Some of the indications of faults are : 



1. Beds formed at different times in' juxtaposition along a 

 plane transverse to their bedding. 



2. Offsetting of formations in outcrop. 



3. Offsetting of outcrops as definite topographic features. 



4. Dikes. 



5. Abrupt changes of strike and dip not indicated in the 

 folds. 



6. Fault breccias. 



7. Slickensides. 



8. Abrupt disappearance of outcrops along a markedly 

 rectilinear boundary. 



9. Occurrence of scarps in the more resistant rocks. 



10. Fault gorges. 



11. Arrangement of surface springs in rectilinear directions. 



It was further realized that it would probably be difficult to 

 locate more than a small proportion of the individual faults 

 occurring within the district, and effort was therefore made as 

 early as possible to learn the distinguishing characteristics — 

 especially the bearings of the individual fault series — of the 



* The Mapping of the Crystalline Schists, I, Methods ; II, Basal Assump- 

 tions. Jour. Geol., x, pp. 780-792, 858-890, 1902. 



