18 Johnson and Warren — Geology of Rhode Island. 



shattered against the ore and the resulting fractures filled with 

 oxides derived from it. The olivine crystals very generally 

 enclose small anhedral ore grains, and are rich in other 

 inclusions. These consist in part of minute black particles and 

 in part of cavities. Many of the latter contain ferruginous 

 matter. While the cavities occur at random through the 

 ciwstals, they are commonly arranged along curious, wavy, 

 ribbon-like lines which do not, so far as observed, follow any 

 crystallographic directions and often extend across more than 

 a single individual. In form the cavities are similar to 

 those noted by various authors in the olivine from other rocks — 

 round, elliptical, or irregular, branching and amoeba-like in 

 shape. Their general appearance and their ferruginous filling 

 point to their being solution cavities formed, perhaps, along 

 old healed-up fracture lines. Many of the opaque particles 

 have the same shape and mode of occurrence, and are, perhaps, 

 cavities entirely filled with ore derived from the matrix. It 

 is possible that others are true crystallites of magnetite and 

 ilmenite included by the olivine. It is noticeable that the 

 strings of inclusions and the material distributed along the 

 fracture lines, like those between the crystals, tend to merge to 

 a greater or less extent into the main body of the ore 

 matrix. Although part, perhaps most of these perplexing 

 inclusions, are probably of secondary origin, it is impossible to 

 escape the feeling that many of them, like the larger included 

 ore grains, represent crystallizations from the magma which 

 were caught by the rapidly crystallizing olivine. 



The mean index of refraction of the olivine was found by 

 the immersion method to be about 1-712, which is considerably 

 above that for ordinary olivine, as is also its specific gravity, 

 3 - 728, as determined by the pycnometer at 20 C. on carefully 

 selected material separated from the crushed rock by magnetic 

 treatment.* 



A chemical analysis (by Warren) of this material gave the 

 following results : 



* Several hundred grams of the crushed rock, from which the finest dust 

 had been removed by washing, was passed through an 80 mesh screen and 

 subjected to a magnetic treatment by which three fractions were made: — 

 No. 1 ilmenite-magnetite mixture; No. 2 olivine; No. 3 feldspar and other 

 non-magnetic materials. Fraction No. 2 was again fractioned with great 

 care, using a magnetic field of varying intensity, until a small fraction of 

 clear yellowish olivine grains was obtained, which showed under the micro- 

 scope almost none of the ferruginous material so abundant in the greater 

 proportion of them. The material thus obtained was used for the analysis. 

 The mean index of refraction of this material was found to be the same (by 

 the immersion method) as that of the olivine taken out with a weaker mag- 

 netic flux because of included ore, indicating that the olivine is of uniform 

 composition. 



