450 J. B. WOODWORTH — VERTEBRATE FOOTPRINTS, PLAINVILLE 



phosed to such an extent that the organic contents of the beds have 

 been largely distorted beyond recognition or actually effaced. 



DISTRIBUTION OF METAMORPHISM 



The contrast between the Wrentham area and the remainder of the 

 basin is very great in regard to the metamorphism of the rocks. This is 

 largely due to the intrusion of pegmatites in successively greater pipes 

 and dikes as we proceed southward from the vicinity of Providence to- 

 ward the mouth of Narragansett bay. In the Wrentham-Attleboro area 

 igneous action has been limited to the intrusion of dikes of diabase, and 

 to the intrusion and effusion of masses of felsites and granophyric rocks 

 which have exerted little or no influence on the strata. Such changes 

 of texture as have been observed in the northwestern corner of the basin 

 are clearly ascribable to lateral pressure, which is manifested in the crush- 

 ing of sediments in closed folds. Where blocks of strata have escaped this 

 action the beds remain without cleavage and without the development 

 of new minerals. 



STRUCTURE OF THE WRENTHAM-A TTLEBORO AREA 



The structure of the area in which fossils have been found is one of 

 combined folding, faulting, and overthrusting in which the sequence has 

 been folding with overthrusting, followed by faulting. About an inlier 

 of pre-Carboniferous granitite with Lower Cambrian beds l} r ing near the 

 northwestern corner of the basin is wrapped a highly folded series of 

 locally lower Carboniferous beds with igneous associates in the form of 

 a horseshoe-shaped area open on the north. Between the arms of this 

 structure on the north of the pre-Carboniferous mass or core lies an 

 overthrust and downfaulted block of locally middle Carboniferous strata 

 tilted westward. The strata on which this block rests are vertical beds 

 of Carboniferous equally high in the series and closely beset with cleav- 

 age. In the overlying block are found the fossils here described. 



Geology of Plainville 



DISTRIBUTION AND CHARACTER OF THE STRATA 



The accompanying sketch map of the vicinity of Plainville, in the 

 southern part of the town of Wrentham, is intended to illustrate the gen- 

 eral features of the fossil iferous locality. The strata consist of an alterna- 

 tion of gray feldspathic sandstones and shales, with occasional massive 

 mill rated mud beds, and one known bed of coal. 



