Ransome. I 



Eruptive Rocks of Point Bonita. 



89 



crystal boundaries. Occasionally, however, there may be observed 

 roughly square sections with suggestions of truncated corners, and 

 elongated sections presumably from the prism zone. The latter 

 exhibit the cleavage in a system of distinct parallel cracks, usually 

 not very numerous, and seldom running the whole length of the 

 crystal. There is also an irregular parting, shown by coarser cracks, 

 transverse to the prism. Sections perpendicular to the c axis gen- 

 erally show a very irregular arrangement of cracks, but the char- 

 acteristic cleavage of augite can sometimes be made out. The 

 extinction in sections parallel to 00 P ^ is about 45 °, and in sec- 

 tions transverse to the prism, bisects the cleavage angles. The 

 index of refraction is high, and the double refraction strong, and 

 positive in character. 



The augite is generally remarkably fresh, and in many slides 

 shows not the slightest trace of decomposition, in which cases it 

 contrasts strongly with the clouded plagioclases. The colors by 

 transmitted light vary, in different specimens, from a nearly col- 

 orless pinkish or greenish white to a deep violet red. Pleochroism 

 is barely discernible or absent in the paler varieties, but becomes 

 very marked in proportion to the increasing depth of color, a being 

 light yellowish green, b light reddish brown, and c a rich violet 

 red. The absorption formula is c > b > a. The deep color and 

 pleochroism being suggestive of the presence of titanium* a portion 

 of the rock containing the most deeply colored crystals was pow- 

 dered, and the augite separated by means of Klein's solution. The 

 purified powder showed, under the microscope, grains of a deep, 

 clear, amethyst color, which were found to give distinct qualitative 

 reactions for titanium. The specific gravity of the isolated powder 

 was determined as 3.439. The foregoing result thus adds new 

 support to the hypothesis first put forth by Knop.f that the violet 



*A. Knop, Ueber die Augite des Kaiserstuhlgebirges, Zeits. f. Krys. u. 

 Min. Groth., 1885, p. 75. 



Rosenbusch, Mikroskopische Physiographie, Stuttgart, 1885, p. 437. 

 Teall, British Petrography, London, 188S, p. 159. 



M. Hunter und H. Rosenbusch, iiber Monchiquit, ein Camptonitisches 

 Ganggestein, etc. Tscherm. Min. u. Pet. Mitth. N. F. b XI, p. 461. 

 t Loc. n't., p. 75. 



