F. H. Lahee — Metamorphism and Geological Structure. 255 



According to the foregoing review of theoretical facts, 

 variations in the intensity of deformation may be due (1) to 

 the type of folding ; (2) to the position of the outcrop in the 

 fold ; (3) to the degree of rigidity of the rock : and, (£) to the 

 distance of the outcrop from the point of application of the 

 force. Provided the proper conditions prevail, then, we 

 should expect to find such variations in the structure of the 

 Narragansett Basin. 



In the description which is to follow, we shall be able 

 neither to mention strikes and dips of individual outcrops * 

 nor to debate the pros and cons of questionable intepretations 

 of the folding, f The method of procedure will be indicated 

 and then the facts will be presented in summary form. 



The Borders of the Basin. — As may be seen on the map, 

 the borders of the Basin have many irregularities of trend. 

 Causes for these changes in direction may be : (1) a pre-Carbon- 

 iferous hill-and-valley topography, forming the floor of the 

 Basin ; (2) the deformation of a more level pre-Carboniferous 

 land surface ; or, (3) a system of post-Carboniferous faults. If 

 the first supposition were true, the Carboniferous sediments 

 should often abut against the pre-Carboniferous, and there 

 should be little dependency between strikes of the strata and 

 strikes of the surface of unconformity separating the Carbon- 

 iferous and the pre-Carboniferous. But such is not the case. 

 There is a remarkably close parallelism between the attitude of 

 this surface of unconformity and the attitudes of the adjacent 

 Basin sediments. Indeed, it is just what would be expected if 

 the Bason floor had been originally comparatively flat and had 

 later shared in the diastrophism of the overlying strata. Fur- 

 ther evidence for deformation of the basement, according to 

 Shaler, is to be seen in a certain amount of schistose structure 

 in the eastern and western border-rocks, which decreases in 

 intensity away from the Basin 4 



The Basin floor has been deformed not only by bending, but 

 also by faulting. This is indicated locally by exceptional 

 straightness of the rim, by apparent displacement of beds or of 

 groups of beds, and by zones of fault brecciation. Whether all 

 of this fracturing is of the normal, or tension, type, or whether 

 some of it is of the reversed, or compression, type, could not be 

 determined. Certainly many of the faults are normal. 



Summarizing, we infer (1) that the original floor of the 

 Basin, which comes to the present topographic level in or near 



*The strikes and dips have been more or less generalized for different 

 localities and have been plotted on the map (fig. 1). 



fFor these see Shaler, N. S., etc., op. cit., and Lahee, F. EL, A study of 

 Metamorphism in the Carboniferous Formation of the Narragansett Basin ; 

 thesis deposited in Gore Hall, Harvard University, in 1911. 



t Shaler, N. S., op. cit., pp. 19-20. 



