P. P. D. Graham — Ojjtical Properties of Hastingsite. 541 



Under the microscope in parallel light, thin sections appear 

 quite fresh and greenish in color, with a very strong 

 pleochroism. Those rhomb-shaped sections, which are cut 

 more or less perpendicular to the prism, and show the two sets 

 of cleavage cracks intersecting at about 56°, are yellowish 

 green for light vibrating along the shorter diagonal of the 

 rhomb, and deep bluish green, or nearly opaque if the section 

 is at all thick, for light vibrating parallel to the longer 

 diagonal. Prismatic sections are also very strongly pleochroic, 

 appearing deep bluish green to opaque when the light trav- 

 ersing them vibrates along the cleavage and pale yellowish 

 green for light vibrating perpendicular thereto. Between 

 crossed nicols the latter have various angles of extinction with 

 the cleavage cracks, the maximum value observed being about 

 30°. 



Some fragments, however, while being distinctly pleochroic, 

 exhibit this property in a comparatively slight degree, and 

 these are further found to be almost isotropic between crossed 

 nicols. When examined in convergent light, a dark cross, 

 somewhat blurred and thickened at its center, is seen, and it 

 was this unusual feature which first drew special attention to 

 the mineral. The cross does not separate into very definite 

 hyperbolas on rotating the section, owing to its ill-defined 

 character, but that the mineral is not truly uniaxial is evident 

 from the pleochroism of these sections and also from the 

 unsymmetrical manner in which the brushes are colored. 



In the paper referred to above, it was stated that the axial 

 angle is over 30° and possibly as much as 45°, the optic axes 

 lying in the plane of symmetry, with a strong dispersion in the 

 sense p > v. 



The optical determination of the mineral in the ordinary 

 rock sections is a somewhat difficult matter, owing to the fact 

 that even the thinnest slices, when cut normal to the acute 

 bisectrix, have a very deep bluish green color, causing the 

 whole field to be dark, while the power of the objective under 

 which it can be examined is also necessarily limited in such 

 cases. In the present instance, small chips of the mineral 

 were crushed very finely under oil and examined under a 1/12" 

 oil immersion objective. The majority of the fragments 

 were minute cleavage flakes, with a high extinction angle, the 

 mean observed value being 22°, and they exhibit the strong 

 pleochroism noted above for prismatic sections. The bire- 

 fringence is low, compensation taking place when the quartz 

 wedge is inserted across the prism. The dark brush which 

 crosses the field on rotating the section (or the nicols) in con- 

 vergent light is broadly fringed with red on one side and blue 

 on the other, indicating a strong dispersion. 



Am. Jour Sci.— Fourth Series, Vol. XXVIII, No. 168.— December, 1909. 

 36 



