TECTONIC GEOGRAPHY 555 



outlined an investigation by newly derived methods for the discovery of fault 

 structures and for the determination of their system within the southwestern New 

 England province. Without here recounting these methods, which have been 

 elsewhere briefly described,* it maj' be stated that the most important respect in 

 which they differ from established methods of attack upon tectonic problems of 

 the crystalline schists lies in the observation of the orientation of topographic and 

 hydrographic features and geologic boundaries. Another departure from estab- 

 lished methods has been to study with detail, which by contrast is almost micro- 

 scopic, very limited areas which are distinguished for their complexity of areal 

 relations. Within these areas the study is intended to develop the essential nature 

 of the deformation of the districts in which they lie — the relative importance of 

 fold and fault structures and the dominant elements in each system — so that they 

 may be regarded in some sense as key areas for the study of the province. 



Five areas of this kind — the Lee (Massachusetts) ridges, the near-lying Evergreen 

 Hill area, the Twin Lakes (Connecticut) Valley area, the Pomperaug (Connecticut) 

 valley, and the Greater New York City area— have now been studied and the domi- 

 nant influence of fault structures established for each. The joint system of each 

 area has also been studied and its relation to the orientation, to the topographic 

 and hydrographic lines (the lineaments), and to the geologic boundaries has been 

 considered. The dominant joint system has been found in each area to be vertical 

 or nearly so, the joint planes falling within a considerable number of parallel series, 

 of which a comparatively small number greatly outweigh all the others in impor- 

 tance. The majority of directions of joint series are found to be in essential agree- 

 ment between the different areas, though certain directions have been observed in 

 particular areas only, and it would appear that in a few cases a series in one area 

 may be replaced by a near-lying series in another. 



A principal aim of the investigation has been to determine whether the fault 

 planes within the area fall into a system in essential agreement with the joint sys- 

 tem, as has been elsewhere determined by Daubree and Brogger. If the methods 

 derived are to be trusted, the study fully confirms the view of these geologists, who 

 have seen in the system of joints the first effect of the deformation of a province by 

 fracture. The author's view is that the release of compressive stress incident to the 

 formation of the planes of separation (joints) reduces the competency of the crustal 

 block to sustain loads, reduces its rigidity regarded as a girder, and allows of the 

 differential movements of its parts along the joint planes, in which the idea of 

 faults inheres. 



Having examined the key areas and having found a combined system of joints 

 and faults in each, the investigation naturally turned in the direction of comparing 

 the lineaments in the districts surrounding these areas with the joint and fault sys- 

 tem made out. The following table indicates the relationships between the linea- 

 ments in these districts and the joint directions which were observed. The district 

 surrounding the Lee ridges and the near- lying Evergreen hill has been designated 

 the Stockbridge valley, that surrounding the Twin Lakes area the Sheffield valley. 



In bringing about the present positions and attitudes of the rock masses, and in 

 producing the present topographic lines, it is concluded that folds, though every- 

 where apparent, have been altogether subordinate to the mosaic produced by the 

 differential movement of orographic blocks. In a large measure this is true because 



♦The mapping of the crystalline schists. Jour. Geol., vol. x, pp. 782-791, 858-890. 



