32 G. H. Williams — Peridotites near PeekskilL JS T . Y. 



variety known as basaltic hornblende. The difference in ab- 

 sorption between the ray vibrating parallel to the axis of great- 

 est elasticit} 7 , a, and either of the others is very marked ; but 

 between the rays vibrating parallel to c and h a difference is 

 hardly observable.* The color of the c ray is a dark chest- 

 nut; that of the h ray the slightest tinge lighter, while a is a 

 light yellow. The absorption may be expressed by the formula 

 c=6$> cuf I n sections therefore which lie in the zone oo P oo : OP 

 a very strong change of color is observed when the stage is 

 revolved over the polarizer. On the other hand sections nearly 

 parallel to go Pot or ooP remain dark brown by a complete 

 revolution. This accounts for the fact that apparently non- 

 pleochroic hornblende often occurs in the same section with 

 such as is strongly pleochroic. The difference depends only 

 upon the direction in which the mineral is cut. The extinction 

 angle in a section cut parallel to the clinopinacoid, measured, 

 against the cleavage lines (vertical axis), gave values varying 

 between 9° and 10 b . 



The inclusions in this hornblende are both numerous and 

 characteristic. The most common are opaque black needles, 

 ranging in size from the finest dust to about "03 mm. in length. 



The majority of these are arranged 

 either parallel to the vertical axis 

 or else so as to make an angle of 

 about 45° with this. Others appear 

 quite irregular in their position. 

 More rarely small transparent crys- 

 tals, the largest of which are *05 mm. 

 long and "02 mm. broad, occur with 

 the opaque needles. The nature of 

 these could not be determined. 

 They seemed, however, to possess 

 a sharp crystal form, a high index 

 of refraction and a parallel extinction. These are most frequently 

 arranged with their longest axes inclined approximately 45° to 

 the c. axis of the hornblende, or perpendicular to it. Still more 

 rarely than these transparent crystals, the hornblende contains 

 inclusions of thin brown plates similar to those which are so char- 

 acteristic of hypersthene. All of these interpositions, of which 

 the opaque black needles as a rule occur alone, show a tendency 

 to concentration toward the center of the hornblende, leaving a 

 border near the edge comparatively free from foreign substances. 



*In the mineral hornblende, i (axis of middle elasticity) coincides with the 



crystallographic axis b (orthodiagonal) ; c (axis of least elasticity) agrees nearly 



i 

 with c (vertical axis), while a, (axis of greatest elasticity and principal bisectrix) 

 is situated in the plane of symmetry at right angles to c. 



■f Vid F. Becke, Tschermak's Min. und Petrog. Mitth., 1882, p. 235. 



