12 Johnson and Warren — Geology of Rhode Island. 



its classification as a syenite by some of the earlier investigators 

 in the region. 



Conclusions as to the relations of the two basic rocks. — The 

 presence of numerous inclusions of the gabbro in the cumber- 

 landite, particularly on its western side, argues in favor of a 

 not very distant contact between the two rocks. _ If we con- 

 nect the north- and southeastern ends of the gabbro as shown 

 on the map, it can be seen that the contact with the cumber- 

 landite would lie near to what, from other considerations, is 

 thought to be the western limit of the latter. If the gabbro 

 extends somewhat farther northeast than its present outcrops 

 show, as is very probable, the line of contact would lie still 

 closer to the present hill. The inclusions of the gabbro in the 

 rock of the hill show clearly the latter 1 is the younger of the 

 two, and that they do not grade into one another, as is the case 

 with so many occurrences of " titaniferous iron ores" associated 

 with gabbroitic rocks. The basic character of both, their large 

 content of ilmenite and their occurrence as adjoining masses 

 entirely surrounded by genetically unrelated rocks is strongly 

 suggestive that they are offshoots from a common magma. 



Part II — The Petrography and Mineralogy of Iron Mine Hill, 

 Cumberland, M. I. 



The Rhodose {Cumberlandite). — Macroscopically the typical 

 rhodose may be described as a rock composed of colorless to 

 white plagioclase crystals, embedded in a black, granular 

 groundmass possessing a metallic to resinous luster and a tough, 

 irregular fracture. The appearance either in the ledge or 

 hand-specimen is highly characteristic and with a specific gravity 

 of almost exactly four the rhodose is exceptional in character. 



On weathering, the feldspar whitens and assumes a chalky 

 appearance. The olivine alters readily to a reddish or brownish 

 ferruginous clay, leaving the black ilmenite and magnetite in 

 granular relief. The weathered layer, once formed, acts like 

 a blanket and serves to protect the rock beneath very effectually 

 from further action. 



The groundmass is easily resolved with the aid of a lens, 

 less easily with the eye alone, into two constituents, one, a 

 yellowish, glassy olivine (hyalosiderite) exhibiting frequently 

 well developed cleavage surfaces, and the other, a grayish black 

 mass of magnetite and ilmenite. In slightly weathered speci- 

 mens the olivine assumes a yellowish green to brownish color 

 and is then easily distinguished by the eye. 



The feldspar . — The plagioclase makes up about 13 per cent by 

 volume of the rock. The crystals have generally a thinly tabular 

 habit parallel to 010, and vary considerably in size from minute 



