454 Hawkins — Notes on the Geology of Rhode Island. 



just south of Jamestown. They are in turn cut by 

 quartz veins. 



Fourth are fine-grained dikes of diabase, usually in 

 vertical or steeply dipping position, and of the type 

 usually referred to the Triassic. The necessary proof 

 of exact age is lacking. They are all in the northern 

 third of the State, occurring, together with the adjacent 

 intrusive sills of Attleboro, Massachusetts, already men- 

 tioned, within an area about 12 miles square, lying to 

 the northeast, north, and northwest of Providence, and 

 including Cumberland Hill and Woonsocket. To this 

 type belong the 60-foot dike at Woonsocket (cutting Car- 

 boniferous sediments), the 10-foot one at Snake Den (in 

 granite gneiss), the 8-inch one at Miner's Crossing 

 (in green schist), the narrow one at Cumberland Hill 

 (intruding gabbro), and others at Wionkhiege Hill, 

 Primrose, and in the cut for the new State road on Law- 

 ton Hill, west of Thornton. Usually these diabasic dike 

 rocks show abnormal mineral constituents resulting from 

 entire or partial solution of portions of acid terranes 

 adjacent. Daly finds this process often occurring with 

 the same result, 23 and Powers 24 has quite fully discussed 

 the nature and origin of such foreign materials. The 

 large diabase dike at "Woonsocket, 60 feet or more across, 

 encloses relatively enormous quantities of angular and 

 rounded fragments of quartz, granite, diorite, and Car- 

 boniferous shale. The inclusions in this dike have been 

 observed to be at times four feet long by two feet wide, 

 and the foreign materials frequently make up as much as 

 50% of the total rock mass. Dikes in the region to the 

 southward and eastward of Woonsocket show similar phe- 

 nomena to a more limited extent. One dike (that at 

 Snake Den in Johnston) is filled with microscopic cav- 

 ities ("Kugel" Structure). 



These rocks are typical olivine diabase, of which one of 

 the best developed, that at Snake Den, has been selected 

 as the type, for microscopical and chemical investiga- 

 tion. A chemical analysis of it is given (see analysis 

 No. 7. Table II). This diabase shows two periods of 

 crystallization during cooling, as is typical of this 

 rock type. The phenocrysts are olivine, labradorite 

 (AbiAnJ, and pyroxene. The groundmass is composed 



23 Daly, E. A., "Igneous Rocks and Their Origin," 1914. 



24 Powers, S., Jour. Geol., 23, 1-10, 166-182, 1915. 



