450 WARREN AND POWERS DIAMOND HILL-CUMBERLAND DISTRICT 



Cambrian seliist and a highly altered and sheared rock which appears to 

 have originally been a gabbru. The latter is intrusive into the schist. 



This gabbro-^ is exposed in a relatively fresh condition only in cuts 

 along a deserted railroad running from Iron Mine Hill to the Woon- 

 socket road, a mile to the west. From this locality it may be traced into 

 a greenisli white schist, the extremely sheared and altered phase of the 

 rock. 



The fresh gabbro is a medium to coarse-grained, greenish to brownish 

 rock consisting originally of plagioclase, augite, ilmenite, magnetite, and 

 apatite. In the least altered phase the plagioclase is a labradorite of 

 about the composition AbiAn^, occurring in tabular crystals. These are 

 comparatively fresh, but are colored brownish by a pigment present 

 within the crystals. The augite has been largely altered to secondary 

 hornblende accompanied by m.ore or less biotite. In this alteration have 

 also been involved the large ilmenite grains originally closely associated 

 with the augite, and the adjacent labradorite crystals. Further altera- 

 tion, accompanied by shearing, has saussuritized the feldspar, altered the 

 hornblende and biotite to chlorite and epidote or zoisite, and scattered 

 these products generally through the rock. The remaining ilmenite 

 formed leucoxene. The- original texture of the gabbro is thus largely 

 destroyed. Further metamorphism has reduced the rock to a compact 

 green schist in which almost all traces of the original rock structure has 

 been lost. For further details of the chemical composition and meta- 

 morphism, etcetera, of this rock the paper above referred to may be 

 consulted. 



The age of the intrusion of the gabbro is placed in the pre-Cambrian, 

 in part by reason of the evidence found west of West Wrentham, as noted 

 above, and in part by reason of the fact that this gabbro is probably a 

 southwestward extension of a band 2 miles wide of very similar rock ex- 

 tending from near Sheldonville through Sharon to Canton Junction, a 

 distance of 14 miles. The gabbro east of Sheldonville is cut by biotite 

 granite and is, therefore, older than the granite.^^ 



Cumberlandite. — The cumberlandite is perhaps the most interesting- 

 rock petrologically which is found in the area, but as it has been fully 

 described by one of the writers a brief summary will suffice here.^^ The 

 i-ather prominent rounded knob known as "Iron Mine Hill" consists 

 entirely of this rock. It is exposed as a roughly elliptical, dikelike boss 



31 See paper by C. H. Warren in Am. Jour. Sci.. vol. 26, 1008, pp. 460-477, 

 ~ This rock has been studied by Mr. W, P. Haynes and also by Mr. G. E. Goodspeed, 

 .Ir.. whose field reports were consulted by the writers. 

 2« Op. cit 



