IGNEOUS ROCKS 451 



having a length of at least 1,200 feet and a width of about 600 feet. It 

 cuts the pre-Cambrian schist on the east and the gabbro on the west. 

 The northern and southern extensions are obscured by drift. The 

 greater part of the mass, as at present exposed, consists of altered phases. 

 The extreme phase is greenish black in color and consists of a dense, fine- 

 grained mass of ilmenite, magnetite, serpentine, chlorite, and actinolite. 

 Its otherwise almost dense texture is relieved by irregular dull green 

 spots of serpentine and chlorite which mark the positions of original lab- 

 radorite phenocrysts. In the less highly altered phases a considerable part 

 of the original olivine remains. The original rock, exposed on the west- 

 ern central side of the hill, consists of a fine-grained groundmass of 

 olivine (hyalosiderite), 40 per cent, and enmeshing magnetite and ilmen- 

 ite, 18 and 20 per cent respectively, crystallographically intergrown, 

 embedding relatively large phenocrysts of labradorite occurring singly 

 and in clusters. The feldspar constitutes about 10 per cent of the whole. 

 In the ore matrix are about 3 per cent of dark green spinal crystals.^* 



In narrow veins traversing the cumberlandite are found crystallizations 

 of actinolite, clinochlore, and hortonolite.^^ Occasional narrow veins of 

 fibrous, brittle serpentine also occur in a few places. Many years ago 

 the cumberlandite, containing, as it does, 30 per cent of iron, was mined, 

 and when mixed with hematite ores from other localities, particularly 

 that of Cranston, Rhode Island, is said to have yielded an excellent qual- 

 ity of iron. It has been more recently exploited, without apparently 

 much success, as a road metal. 



The cumberlandite is known to be younger than the gabbro on the 

 west. Further than this, there is no evidence concerning its age. It was 

 probably intruded in later pre-Cambrian times and may be a differentiate 

 of a pre-Cambrian magma. It may, on the other hand, be of much later 

 date. 



Serpentine veins. — In a field west of Iron Mine Hill, underlain by 

 either granite or gabbro, probably by the former, there are several large 

 angular boulders of dense, bluish gray serpentine which appear to be 

 almost in place. These boulders form a mass about 5 feet in width, 10 

 feet in length, and 5 feet in height. Dr. C. T. Jackson in 1840 reported 

 serpentine to occur at this locality in the form of a dike. It is probable 

 that these boulders are in place, but that they have been blasted to make 

 the field suitable for ploughing. The serpentine to the west of the hill 



2* Rounded boulders of this rock are abundant in the drift to the southward of Iron 

 Mine Hill, and when deeply altered and covered with rust, as they usually are, they 

 suggest very strongly a meteorite in appearance. Such boulders are, in fact, frequently 

 mistaken for meteorites. 



28 C. H. Warren : Z. K. No. 19. 



XXXII — Bull. Geol. Soc. Am., Vol. 25, 1913 



