Johnson and Warren — Geology of Rhode Island. 15 



composition must, therefore, be also small. No orthoclase has 

 been observed. 



Along the fractures in the feldspar crystals some alteration 

 material is noticeable. This consists largely of actinolite mixed 

 with some kaolinite and a little calcite or dolomite ; otherwise 

 the feldspars are quite fresh. Where the plagioclase comes in 

 contact with the groundmass, a characteristic reaction rim is 

 always developed. The reaction material consists essentially 

 of pale green to colorless actinolite needles orientated perpen- 

 dicular to the contact. When in contact with the ilmenite- 

 magnetite, shreds of biotite frequently make their appearance 

 and sometimes constitute the entire rim. The biotite often lies 

 next the ore, as a narrow border, and a change into actinolite 

 may sometimes be seen on the side towards the feldspar. The 

 width of the reaction rims appears to be somewhat greater about 

 the ore than about the olivine, but in any case they are exceed- 

 ingly narrow, varying between approximately •003 mm and '01 mm , 

 rarely exceeding the latter figure. The rims are too narrow to in 

 any measure account for the rounded character of the feldspar 

 outlines. The formation of the reaction rims appears to have 

 been quite distinct from the more extensive metamorphic altern- 

 ation which replaced the feldspar with chloritic material. 



A few crystals of biotite, exactly like the biotite shreds 

 associated with the actinolite in the reaction rims, are met with 

 throughout the rock. They are always associated with the ore and 

 are clearly secondary. This suggests, at least, that the biotite 

 observed in the closely related rock from Taberg, Sweden, may 

 also be secondary. 



Enclosed in one or two of the feldspar crystals, several irreg- 

 ular grains, of a colorless to brownish, highly refracting, 

 isotropic substance were observed. They have not been posi- 

 tively identified, but are thought to be garnet. 



The groundmass. — A polished surface of the rock serves 

 admirably to bring out the texture of the groundmass. The 

 ore, if looked at directly, possesses a grayish black color, but if 

 viewed at an angle, shows a peculiar and characteristic bronze 

 tint. It forms a more or less continuous network through the 

 rock and serves as a matrix for the olivine. No crystallographic 

 outlines have been identified with certainty in the ore. The 

 olivine occupies somewhat more than one-half of the surface, 

 possesses a dark, glassy appearance, in sharp contrast to the ore, 

 and is characteristically rounded in outline. 



The ore. — On etching the polished surface with hydrochloric 

 acid, a delicate reticulate structure is developed in the ore. 

 With a direct illumination under the microscope, this structure 

 is seen to consist of an intersecting series of bright gray, narrow 

 bands or lamellae enclosing dull black depressions. This struc- 



