4 N. L. Bowen — Genetic Features of 



difference between the two olivines in this particular to 

 permit the determination of the refractive indices. If 

 the rock is powdered and the measurement of indices by 

 the immersion method is undertaken, one usually cannot 

 be sure to which olivine an individual grain belongs. In 

 thin section, however, there is no confusing them, on 

 account of the structural difference mentioned, so a slide 

 was uncovered, washed free from balsam and the rock 

 slice itself used in immersion liquids. Thus the indices 

 of the monticellite were found to be y = 1.668 ; <* = 1.653 

 ± .002. Direct measurement of the maximum birefrin- 

 gence gave 0.015, so that while measurements so made are 

 not of the highest accuracy, it appears that the birefrin- 

 gence is slightly less than that in the Magnet Cove 

 mineral. The Magnet Cove mineral contains nearly 5 

 per cent FeO, and in view of the close approach in indices 

 the present mineral must have nearly the same com- 

 position. Pure artificial monticellite has distinctly lower 

 indices. 9 



The chrysolite has already been described in part, 

 while contrasting it with monticellite. Its refractive 

 indices, measured as above, are y = 1.700 ; a = 1.665 

 ± .003 and 2V close to 90°. It probably contains in the 

 neighborhood of 10 per cent FeO, though the effect of a 

 possible Ti0 2 content is not known. 



The augite needs but brief mention. Its extinction 

 angle mounts to about 45° and the refractive index 

 y=1.715. This combination of properties rather excludes 

 any significant alkali content and fixes it as ordinary 

 augite. It is slightly brownish to greenish in thin section 

 and the typical cleavage is well developed. 



The melilite is in comparatively small grains that show 

 the typical tabular development parallel to the base, 

 though they are not idiomorphic. The basal cleavage 

 is sometimes marked only by the development of minute 

 seams of an alteration product of high birefringence. 

 Occasionally it may be well developed and accompanied 

 by prismatic cleavage of nearly equal strength. The 

 melilite itself always has very low birefringence but is 

 distinctly variable in optical properties. This variation 

 appears to be connected with the amount of melilite in 

 the rock. Where present in small amount it is optically 



9 Ferguson and Merwin, this Journal, (4) 48, 92, 1919. 



