28 Johnson and Warren — Geology of Rhode Island. 



inclusions of the olivine, which have, however, undergone more 

 or less rearrangement and recrystallization. Between the 

 olivine, which is now very often somewhat discolored, and 

 the ore, and sometimes between the olivine crystals where 

 these are near the ore, there is everywhere developed a 

 narrow band of chlorite crystals. The continuity of the ore 

 matrix is now somewhat broken, probably along the 

 boundaries of individual grains, and the breaks are filled 

 with chloritic material. The spinel is also replaced by 

 chlorite, and can be seen in various stages of alteration. 

 Although the ore has clearly contributed to the formation of 

 the secondary minerals, the olivine and the feldspar have fur- 

 nished bj T far the greater portion of the materials. 



Altered Types — "Chloritic." — Beyond the transition type 

 we find the plagioclase phenocrysts entirely replaced by the 

 dark green, amorphous material. The greater part of the 

 olivine is still unaltered, and is easily detected macroscopically. 

 Crystals of a pale green to yellowish actinolite are now conspic- 

 uous in the rock, and are particularly numerous in the 

 neighborhood of the psendomorphic areas of chloritic material. 

 Freshly broken specimens of the rock have a greenish 

 black color, broken by the occasional bright cleavages of 

 olivine, or actinolite, and indistinctly mottled by the chloritic 

 areas. On smooth glaciated surfaces recently cleared of soil, 

 the appearance of the thickly scattered chloritic spots whitened 

 by exposure is strikingly suggestive of the cumulophyric 

 texture of the original rock. Surfaces long exposed to 

 weathering become deeply pitted where the soft chloritic spots 

 wear awaj T and the olivine rusts out, leaving the ore in relief. 

 This type, .which we will designate the ""chloritic type" for 

 convenience, as near as can be told from the exposures, makes 

 up the greater portion of the present hill. It is most typically 

 exposed in the large old quarry on the western side. 



An examination of thin sections of this type shows that the 

 first formed product of the alteration has entirely disappeared. 

 Different psendomorphic areas 'differ more or less in detail, but 

 all consist essentially of chlorite and a variable amount of 

 actinolite in the form of minute prisms. The latter are apt to 

 be segregated particularly toward the center, which then gives a 

 lighter color to the green areas in the hand specimen. The 

 chlorite crystals vary considerably in size, but are on the 

 average larger than in the previous type and show their dis- 

 tinctive properties more clearly. A well-developed cleavage 

 may be seen and many crystals are twinned. There is rarely 

 a feeble pleochroism visible. Some areas consist throughout 

 of a finely thatched mass of chlorite, others consist of a granu- 

 lar aggregate of chlorite and actinolite centrally, surrounded 



