1878.] "• [Frazer. 



and as usual are characterized by their tendency to separate into two or 

 more parts, colored respectively light brown and pale blue. 



The four masses of pyroxene are equally distinguishable by the peculiar 

 net work of clefts which cover surfaces, which in polarized light present 

 usually one of the tints here distributed among them. It is often the case 

 when a thin slide has been carefully and evenly made, that at a certain posi- 

 tion of the analyzer all the labradorite divides itself into its two character- 

 istic colors depending upon the positions of the optical axes of a pair 

 or a series of the twins. With pyroxene it is different. The fragments, in 

 these traps at least, rarely show definite crystalline form, and in any given 

 position of the analyzer there may be found specimens exhibiting any of 

 the indefinitely large; number of gradations in color between bright green 

 and dark violet which accompany the rotation of the analyzer through the 

 angle which separates the projection of their optical axes. 



Only a single hexagonal section is given in the picture, but these figures 

 are distributed, though not profusely, throughout the mass. This is proba- 

 bly a minute column of Apatite, and the low percentage of Phosphoric 

 Oxide in the accompanying analysis sufficiently explains why these forms 

 are comparatively rare. 



A number of these hexagonal forms having been examined, it was found 

 that when most symmetrical they exercised no influence on polarized light, 

 and were, therefore, sections perpendicular to the optical axis of an hex- 

 agonal crystal since the basal plane assumed this form. 



But where the hexagons were distorted, or in the frequent cases where 

 they were covered by a film of vitreous pyroxene or labradorite, the ex- 

 tinctions were more or less irregular. 



One of the quasi-hexagons measured 0.048 mm. between the parallel 

 edges. 



The comparatively large rhombic figure is in all probability a section of 

 calcite parallel to one of the planes of the rhomboheclron. Several of these 

 figures were measured and examined. One of them was 0.11 millimeter 

 in the longer axis. The angles as measured were 81° 03' and 98° 36' re- 

 spectively. This crystal showed four positions of maximum transparency, 

 and four positions of extinction alternating with each other at distances of 

 45°* Another and the largest similar section which was measured showed 

 a longer axis equaled to 0.25 mm. 



An examination under the improved Fuss's microscope with a magnify- 

 ing power of 275 diameters gave : 



Acute angle 80° 30' | 



Obtuse angle 99° 30' J 



In this case there was no general extinction of light during one revolu- 

 tion with or without the quartz prism. 



*The microgoniometer having been set at zero when the Nicols wei-e crossed, 

 the succeeding positions of extinction were very nearly 0°, 90' 180", and 270° 



PROC. AMER. PHILOS. SOC. XVIII. 102. M. PRINTED .IAN . 25, 1879. 



