456 Hawkins — Notes on the Geology of Rhode Island. 



batholith, and through subsequent dias trophic action 

 have been modified greatly in distribution and shape, and 

 in some cases in texture, mineralogical composition, and 

 appearance. Exposures now available show them to be 

 nearly vertical strips of lenticular sheets, lying in the 

 granite gneiss parallel to its gneissic strike and dip 

 throughout. This is to be seen at Westerly and at White 

 Eock, four miles to the north of the latter place, in a 

 small granite quarry. In plan they are evidently of 

 lens- or pod-shaped form, similar to that assumed by 

 deposits of metallic sulphides in the ancient crystalline 

 rocks, as the result of diastrophism ; transverse faults 

 dislocate the series. (Compare the relative positions of 

 the Ironstone gabbro masses, the fault of similar direc- 

 tion and throw which traverses the gabbro mass at 

 Preston, and the fault with horizontal dislocation of 

 approximately two miles which passes through the 

 vicinity of Greenville and Wallum Pond.) The phe- 

 nomena so observed are closely similar to those recorded 

 by Fenner 29 as occurring in northern New Jersey. This 

 also indicates the relationships and probable shape and 

 extent of the well-known peridotite of Cumberland Hill, 

 which, as an included fragment rather than a true dike 

 or stock, still may be regarded as having a considerable 

 downward extent. Its close relationships with the other 

 gabbros of the series are more fully discussed below. 



The larger gabbro masses of western Rhode Island 

 may be one or two miles long and nearly as wide. The 

 rock of these masses is homogeneous, coarse, and almost 

 entirely untouched by either intrusion or shearing. 

 Apparently it is resistant to both agencies, and the 

 granitic intrusions have failed to penetrate far into it. 

 The texture of the gabbro is generally allotriomorphic, 

 and similar to that of Preston, though certain phases are 

 porphyritic, as at Moosup Valley, and diabasic, as in 

 parts of the Ironstone Reservoir mass and at Woon- 

 socket. Along planes of shearing it becomes hornblende 

 or biotite schist, as in the exposures shown south of Pas- 

 coag, on the west side of the reservoir. From the larger 

 gabbro masses smaller ones, usually in the form of 

 long lenticular strips (see map), have separated; their 

 present position and appearance presumably is due to a 

 combination of contact action (stoping) and diastro- 



^Fernier, C. N., Jour. Geol., 22, 594 et seq., 1914. 



