Johnson and Warren — Geology of Rhode Island. 11 



granite, near '.Norwich, Conn., distant some eighty miles in a 

 southwesterly direction from Cumberland, has shown that the 

 gabbro was intrusive into the schist but earlier than the granite, 

 which evidently found its way through the easily ruptured 

 schists, but could not penetrate the more massive and resistant 

 gabbro. This gabbro was intruded into the sediments prior 

 to regional metamorphism, during which it appears to have 

 acted as a resistant plug against which the surrounding rocks 

 were compressed. It seems not unlikely that similar relations 

 are true in the case of the gabbro and cumberlandite, both of 

 which have clearly suffered from extensive metamorphism. 



Finally it is to be noted that the gabbro extends around to 

 the northeast and reaches a point about northwest of Iron 

 Mine Hill. 



The mineral composition of the gabbro is of especial interest 

 in connection with the latter's relation to the cumberlandite. 

 A study of the rock in the field and laboratory show that it 

 was originally a rather coarse (millimeter) grained rock, con- 

 sisting of labradorite (averaging about AbjAnJ, a purplish 

 augite of distinctly diabasic habit, an unusual amount of 

 ilmenite in the form of large conspicuous grains closely 

 associated with the augite and accessory apatite. Olivine or 

 pyroxenes, other than augite, and original hornblende are 

 entirely lacking. Original magnetite was very sparingly 

 present if at all. Shearing and metamorphism appear to have 

 affected the rock quite unevenly. The central and southern 

 portions are the least affected. On the eastern side, along the 

 ridge facing the hill and in the ledges which mark the north- 

 eastern extension of the mass, the rock is profoundly altered. 

 The feldspar has gone over to a dull white saussuritic sub- 

 stance, the secondary hornblende and biotite formed from the 

 first alteration of the augite and ilmenite have altered almost 

 completely to chlorite and epidote, and the remaining ilmenite 

 has changed to leucoxene. The rock is now somewhat schis- 

 tose, the original texture having practically disappeared so far 

 as its macroscopic appearance is concerned, although it is easily 

 made out microscopically. The rock has assumed a greenish 

 white color in striking contrast to the dark, greenish brown of 

 the least altered forms. Various gradations between these 

 forms can be seen in the field. Toward the granite and. schist 

 on the northwest, the gabbro has been severely sheared and 

 has become a fissile green schist, while in the extreme north- 

 east extension of the gabbro it has passed into a pale greenish 

 white, schistose rock composed of finely crushed feldspar, 

 saussuritic products, chlorite, epidote, leucoxene and some 

 sericite. The light color and feldspathic appearance of the 

 more highly altered forms of the gabbro doubtless account for 



