PTEOXENE-ANDESITE. 349 



largest of which are so crowded with inclusions of foreign matter, with only a narrow 

 border of pure feldspar, as to appear decomposed in the hand specimen. The smaller 

 individuals of feldspar are quite free from like inclusions. Besides these are well- 

 developed crystals of pale yellowish green, strongly pleochroic pyroxene; dark brown 

 hornblende with the characteristic black border in not so sharply outlined forms; 

 and, as accessory minerals, magnetite and apatite, with very rarely quartz, mica, 

 zircon, and tridymite. 



The phenocrysts of feldspar, including all that do not take part in the ground- 

 mass, are plagioclase. The largest individuals, reaching 4 mm in length, are in 

 crystals nearly equally developed in the direction of the three axes, and show in the 

 sections, besides crystal faces, rounded outlines. They are not abundant in the rock 

 sections and can not be so carefully studied optically as the smaller feldspars, but 

 from those that are met with it appears that they are not more basic than labradorite, 

 and because of the great amount of glass included in them their separation and 

 chemical analysis would be both difficult and uncertain. The smaller macroscopic 

 individuals have well-defined crystal forms. Their sections are four, five, six, and 

 eight sided and correspond to those cut from crystals with OP, ooPa, 'P, ooP', 

 2 'Fob, 2 P'<5fc faces. They are for the most part prisms, lengthened in the direction of 

 the brachydiagonal, though some appear tabular in the plane of the brachypinacoid. 

 Irregularly outlined fragments are seldom met with. A very marked, sharply defined 

 zonal structure is common to most all the larger crystals, but is wanting in the more 

 minute ones of the groundmass. The cleavage parallel to the base and brachypinacoid 

 is not very generally present nor very perfect, the feldspar having the irregular frac- 

 ture and glassy appearance of sanidine, a resemblance still more striking because of 

 the nearly total absence in half the individuals of polysynthetic twinning, though in 

 almost every instance an apparently simple individual or Carlsbad twin is found to 

 contain one or more thin lamellae of feldspar twinned according to the albite law or 

 to that of pericline. The medium sized individuals seen in the thin sections, which 

 correspond to the smallest feldspars noticed in the hand specimens, from 0-5 mm to 

 l-0 mm in length, show the characteristic polysynthetic twinning of plagiodnse 

 and give angles of extinction symmetrical to the composition plane as follows: 

 15-15, 30-31, 33-33, 33-34, 36-39, which, from the table of extinction-angles 

 published by MM. Fouque et Michel-Levy, 1 correspond to those of anorthite or a 

 feldspar more basic than labradorite. The smaller individuals are twinned after the 

 Carlsbad law, with very few exceptions, and are characterized by having but few 

 lamella;, of short length, lying in two directions at nearly right angles, twinned the 

 one after albite parallel to the brachypinacoid, the other after pericline parallel to the 

 basal cleavage when present. lu many instances the lamella? are entirely wanting, 

 as just noticed. A careful study of all the sections that showed cleavage, or were 



' Fouqu^ et Michel-L6vy. Mim-ralogie Micrographique, p. 228. Paris, 1879. 



