HORNBLENDE MICA -ANDESITE. 365 



form must have been quite rounded as i t partially fused. The cleavage in these feld- 

 spars is very imperfect, and is for the most part wanting, the crystals being irregu- 

 larly cracked like sanidine. The polysynthetic twinning after albite and i>ericline is 

 very unevenly developed. The latter, never repeated to any great extent, is present 

 in many individuals, the lamella; seldom traversing the entire width of the crystal; 

 those produced by the, former twinning vary greatly both in breadth and length in 

 the same individual, as well as in different ones, the feldspars in general being char- 

 acterized by a paucity of striations. This is well shown iu Figs. 3 and 4, PL v, and 

 Fig. '2, PI. vi, the two figures on PI. v, also illustrating the characteristic difference 

 between the largest of the phenocrysts (Fig. 3, PI. v), and the medium sized ones 

 (Fig. 4, PI. v), both being magnified to the same extent, 35 diameters. The largest 

 have quite irregular outlines and an abundance of stria;, while the medium sized feld- 

 spars are very sharply crystallized and are poorly striated. Besides the multiple 

 twinning, nearly every individual is twinned in halves, either after albite or in a 

 manner corresponding to that of Carlsbad in orthoclase; and frequently several indi- 

 viduals have formed in parallel orientation with the brachypinacoid as the plane of 

 contact (Fig. 2, PI. vi). The angle of extinction averages about the same in each of 

 the thin sections studied. By far the larger number of readings give angles ranging 

 from 15 to 31, some being lower and a very few being higher; for example, in thin 

 section 35 the observed angles in the zone perpendicular to the brachypinacoid are 

 7, 150, 20, 200, 210, 210, 280, 300, 31, 320, 350, 350, 4<P. In the other thin sections 

 the higher angles are even scarcer and belong to very perfectly rectangular sections 

 with few striations. Anorthite is probably present only in small amounts, the greater 

 number of the porphyritical feldspars being labradorite. The lath-shaped microlites 

 of feldspar in the groundmass are fibrous and twinned, and have angles of extinction 

 varying only a few degrees from zero and are for the most part oligoclase; the species 

 of the ill defined patches or grains of feldspar in the groundmass is optically indeter- 

 minable. The large crystals of feldspar contain numerous small colorless glass inclu- 

 sions, each with a single gas bubble ; what sometimes appear like particles of dust 

 are found under a high power to be aggregates of these sharply defined inclusions 

 0- . 02 mm in diameter ; they are occasionally arranged iu systematic order, but more com- 

 monly are scattered irregularly through the crystal; some are attached to needles of 

 apatite. There are also inclosed a few microlites of apatite, some of which in turn 

 contain glass inclusions, and rarely small crystals of zircon. The substance of the 

 feldspar in the thin sections studied is absolutely fresh. 



The hornblende is found only in macroscopic crystals affording the character- 

 istic six sided cross sections and having in every case a black bonier, except in the 

 green variety of hornblende-[mica|-andesite at the east base of Hoosae Mountain i4_'. 

 42). The substance of the hornblende is entirely decomposed, not a single unaltered 

 fragment having been found in any of the thin sections. The resulting product is a 



