256 F. H. Lahee — MetamorpJiism and Geological Structure. 



the pre-Carboniferous borders, had a relatively flat, or at most 

 a gently undulating, surface ; and (2) that this surface under- 

 went deformation, both by folding and by faulting, in com- 

 pany with the superjacent strata. 



The Basin Steata. — In drawing conclusions with respect 

 to the structural relations of the Carboniferous strata, consid- 

 erable latitude of interpretation is inevitable. Among, other 

 reasons this is partly because plant remains, although common, 

 have not yet been proved to be of value in this region as indices 

 of stratigraphic horizon, and furthermore, because the entire 

 series of shales, sandstones, and conglomerates, of the Basin, 

 while characterized by numerous textural variations, is so sim- 

 ilar throughout that mineral composition and lithologic struc- 

 ture are almost worthless for correlation. In general, however, 

 a broad sequence has been made out, passing upward from 

 basal conglomerates and arkoses (Pondville arkose, Natick con- 

 glomerate, etc.), through a great thickness of conglomerates, 

 sandstones, and shales with some coal seams (Kingstown and 

 Aquidneck series), to an overlying coarse conglomerate (Digh- 

 ton and Purgatory conglomerates). 



The deformation of these sediments has produced folds of 

 various sizes and shapes. We shall make a distinction between 

 major folds, which are sufficiently large and important to be 

 represented in an ordinary generalized vertical section, and 

 minor folds, which would usually be omitted from such a 

 section. By contortion we mean complex minor deformation 

 in which the strata are bent into closed or overturned folds, or 

 are otherwise severely compressed. Upon this arbitrary classi- 

 fication we shall base the succeeding description. 



Major folding. — Evidences for variations in the major folds 

 may be observed in such factors as the direction of strike, the 

 degree of dip, the direction and amount of dip of axial planes, 

 and the direction and amount of pitch, if there is a pitch. 



Strikes — The greatest regularity of strikes (between 5° and 

 20° E. of 1ST.) occurs in the Kingstown formation, (1) in the 

 western coast belt — between the western border and the west- 

 ern coast of Narragansett Bay — on Boston Neck (B : 15)* and 

 northward to Barber's Height (B : 12), and (2) in northern 

 Conanicut Island (D : 10-13). That is to say, great uniformity 

 is found only in the southwestern portion of the Basin where 

 dips, as a rule, are rather steep. Such parallelism signifies that 

 the maximum component of force was here practically supreme. 



Somewhat less uniformity of direction is shown between 

 Hamilton (Loc. 8, B:ll) and East Greenwich in the western 

 coast belt, and also on Prudence Island. 



* The map (fig. 1) is coordinated by letters and figures along the margins. 

 Some localities are numbered. These will be indicated as follows in the 

 text : Loc. 10, C : 13, i. e., Locality 10 in coordinate square, C : 13. 



