Hawkins — Notes on the Geology of Rhode Island. 453 



mile, with a width varying from one or two inches to 50 

 or 60 feet. 



The diabasic dikes are of four distinct ages, as follows : 



First, dark-colored dikes, now hornblende schist, in the 

 Harris quarries at Limerock. 20 This type cuts limestone 

 of supposedly Cambrian or pre-Cambrian age (Smith- 

 field limestone), and is in turn intruded by fine-grained 

 aplitic granite offshoots, which probably belong to the 

 adjacent Milford granite series. This hornblende dike 

 material is the "ocLinite" of Emerson and Perry (op. 

 cit.), and is immediately post-shearing. 



Second are the diabasic sills of South Attleboro, Mas- 

 sachusetts, 21 just across the Rhode Island line, which 

 rarely appear as dikes. They are somewhat later than 

 the red Carboniferous beds of the Attleboro area (Wam- 

 sutta group), being perhaps post-Carboniferous. But 

 these traps, which cut felsites, are now found by C. W. 

 Brown to be definitely cut by felsites of another age, a 

 relationship which has not been shown in the earlier 

 work (op. cit.). 



Third are placed the minette, or "mica trap" dikes, 

 apophyses corresponding to mica syenites. Dikes of this 

 rare rock have been described as occurring on Conanicut 

 Island, R. I. 22 The latter writer, in the paper just cited, 

 gives an analysis of the rock (here quoted, see analysis 

 No. 9, Table II). There is also a new dike, of the same 

 nature, now to be reported from Sakonnet Point, some 

 fifteen miles farther east. This is a five-foot dike with a 

 strike of N. 30° W. and a dip of 75° W. ; it cuts coarse 

 red granite of unknown age. Attention was originally 

 called to this dike by Mr. H. I. Richmond. The deter- 

 mination of phosphoric acid in the minette of Conanicut, 

 here stated for the first time (see analysis No. 9, Table 

 II), shows it to contain only a very small amount. The 

 minette of Sakonnet, however, is unusual in containing 

 about 12% of apatite, forming large idiomorphic crystals 

 which are very prominent in the microscopic section. 

 (Also .see analysis, No. 10, Table II.) These minettes 

 have been described as cutting granites and Carbonifer- 

 ous sediments at Conanicut. They have also recently 

 been found to intrude the green schists, at a point 



20 See Emerson (idem, 185). 



21 Shaler, Woodworth, and Foerste, op. cit., 152. 



22 Collie, G. L., Trans. Wis. Acad. Sci., 10, 36, 1895. 



