Johnson and Warren — Geology of Rhode Island. 31 



identical, and no great error can be made by looking upon 

 the composition of the vein actinolite, given beyond, as repre- 

 senting closely that of the mineral from the rock. 



Serpentine type. — Both the chloritic and the actinolitic 

 types by the replacement of the remaining olivine, or of the 

 actinolite, by serpentine, pass gradually into what will be called 

 the serpentine type. This represents a more advanced stage 

 in the alteration and is the predominant type at the northern 

 end of the hill. It is also found along the eastern side, 

 locally on top, and was probably characteristic of the 

 peripheral portions of the mass. In appearance it resembles 

 the chloritic t}?pe, from which it can be distinguished by its 

 somewhat greener color and by noting with a lens the pres- 

 ence of a leek-green, foliated serpentine in place of the 

 original olivine of the groundmass. The ore preserves its 

 characteristic appearance, but has a more noticeable bronzy 

 tint on broken surfaces than the unaltered rock. 



Thin sections show that the chlorite and actinolite of the 

 pseudomorphic areas are much the same as in the other types, 

 although the crystals of the chlorite are commonly larger and 

 occasionally show a delicate plumose texture while the actinolite 

 is more strongly segregated. A variable amount of finely 

 crystalline serpentine and ore, the latter both residual or second- 

 ary, is now mingled with the other constituents. The areas 

 themselves appear to have encroached on the groundmass more 

 extensively than elsewhere except perhaps in extreme cases of 

 the actinolitic replacement. 



The olivine is seen in all stages of replacement by a finely 

 lamellar to fibrous serpentine having a beautiful divergent tex- 

 ture. The serpentine starts at or in the chloritic borders and 

 shoots out at random into the olivine grains, which here are 

 filled with a brown dust. The serpentine shows gray polariza- 

 tion colors, a parallel extinction and a positive elongation. 

 The actinolite crystals seem to have undergone replacement 

 in the same manner as the olivine. In most of the slides 

 examined considerable residual olivine and actinolite remain 

 in the interstices between the serpentine lamellae, and form, with 

 their brilliant polarization colors, a striking contrast to the dull 

 gray serpentine. 



Although the ore matrix has suffered some absorption aucl 

 removal during the serpentinization, and its continuity has been 

 more or less broken, it still retains its original outlines and 

 therefore preserves to a remarkable degree the peculiar texture 

 of the unaltered rock. Few rocks if any that have come under 

 the writer's observation show phenomena of alteration and 

 replacement in so beautiful and convincing a manner. Figure 



