26 Johnson and Warren — Geology of Rhode Island. 



A computation of the specific gravity of the rock, from its 

 percentage composition and the specific gravities of the indi- 

 vidual minerals given above, gives 4/003, which is in close 

 agreement with the figure obtained experimentally, 4*005, by 

 J. E. "Wolff, quoted by Wadsworth, and of 3'92 by the writer. 



With regard to the volume percentage (see above table) of 

 the different minerals, it is to be noted that the olivine makes 

 up one-half of the volume of the rock, while the magnetite 

 and ilmenite occupy, relative to each other, almost exactly the 

 same space, and make up a total of 31*1 per cent. In the last 

 column of the above table is given the percentage compo- 

 sition of the ilmenite calculated to 100 per cent. 



Transition Type. — In going either north along the ledge 

 from the exposure of the unaltered rock, or downward and to 

 the west (outward), within a few yards one comes upon what 

 is a transition into the more highly altered types of the hill. 

 The transition zone seems to be of variable width, but never 

 exceeds a few yards in actual measure. The rock is distin- 

 guished macroscopically from the original by the presence of 

 a dark green, amorphous border of alteration material about 

 the plagioclase crystals. This border is the first indication of 

 a change, and as one goes farther from the unaltered rock 

 it grows broader, spreading out irregularly into the ground- 

 mass, while at the same time the core of feldspar grows 

 smaller. 



As seen in thin sections without crossed nicols, the alteration 

 border is apparently homogeneous except for occasional specks 

 of actinolite or ore, and differs in appearance from the core of 

 unaltered feldspar in being less transparent, in possessing a 

 pale yellowish-green discoloration, a somewhat higher index of 

 refraction and consequently a different surface. That its index 

 is higher than the feldspar was proved by observing the total 

 reflection phenomena at the contact between the two after the 

 method developed by Van der Kolk and elaborated by F. E. 

 Wright- and also by the well-known method of Becke. Be- 

 tween crossed nicols the border is resolved into two distinct 

 substances. 



One is seen directly in contact with the feldspar and is clearly 

 the first product formed. The structure is more or less confused 

 but may be described as semi-compact to sub-fibrous and often 

 shows a divergent structure (plumose). Distant from the feld- 

 spar the structure becomes still more confused but at times 

 approaches a granular texture. As near as can be told, the ex- 

 tinction is parallel to the elongation, as is also the direction of 

 vibration of the slower ray (C). The interference colors have 

 never been noted higher than yellow of the first order in sec- 



* This Journal, No. 167. vol. xvii, pp. 385-7, 1904. 



