10 ilR. J. A. DOUGLAS OX GEOLOGICAL SECTIONS [vol. lxxvi, 



intensely crushed and broken, the cracks being filled with a greenish 

 alteration-product which appears to be a variety of serpentine. 



Numerous colourless crystals of kyanite are also present, showing the 

 characteristic cleavage and high index of refraction. 



Magnetite occurs in somewhat large irregular grains. 



(A 91) Granite. Mejia (kil. 15). (PI. II, fig. 2.) 



Microscopic characters : — A coarse-grained noncrystalline rock with 

 typical granitoid structure. It consists chiefly of quartz and felspar, 

 to which the coloured minerals are quite subordinate. 



The felspars comprise microcline in large crystals with characteristic 

 cross-hatching, orthoclase, and plagioclase ; monoclinic and triclinic 

 forms frequently showing perthitic intergrowths. The plagioclase 

 exhibits very narrow twin-lamella?, and has a refractive index consider- 

 ably lower than that of balsam, being nearer albite. Its decomposition 

 has resulted in the production of flaky aggregates of sericite ; while, in 

 some instances, the entire centre of a crystal is replaced by a clear patch 

 of brilliantly-polarizing muscovite. 



A little biotite is present, but it has been almost completely converted 

 into green chlorite. 



The minor accessories include apatite, magnetite, hasmatite, and small 

 crystals of zircon, the last-named occurring as inclusions in the biotite 

 with strongly pleochroic halos. 



In addition, there are frequent pseuclomorphs of calcite crowded with 

 small yellow crystals, which from their outlines and extremely high re- 

 fractive index appear to be anatase. These pseudomorphs probably 

 represent original sphene. 



(A 103) Pegmatite. Mollendo (kil. 2). 



Microscopic characters : — This rock consists essentially of quartz and 

 microcline in coarse pegmatitic intergrowth. 



Coloured minerals are almost entirely absent, but muscovite occurs in 

 some abundance, largely in the form of sericite from the decomposition 

 of the felspar. 



(A 101) Greisen. Mollendo (kil. 2). 



Microscopic characters : — The bulk of this vein consists of an irregular 

 mosaic of clear quartz and brilliantly-polarizing muscovite, although 

 there is also a certain amount of much- decomposed felspar. This appears 

 to have been chiefly orthoclase, but traces of albite-twinning are occa- 

 sionally visible. 



A little biotite, showing conversion into chlorite, is also present. 



(A 70) Hornblende - mica-diorite. Mollendo (kil. 1). 



Microscopic characters : — A holocrystalline rock of medium grain, con- 

 sisting chiefly of felspar and ferromagnesian minerals (hornblende and 

 biotite) in roughly equal proportions. 



Quartz occurs in some abundance, but appears to be more or less re- 

 stricted to definite veins, and is therefore probably in great part 

 of secondary origin ; analysis of the rock would undoubtedly give a 

 somewhat high percentage of silica. 



The felspar is chiefly plagioclase, showing twinning on the albite law ; 

 the extinction-angles of sections, cut perpendicular to the lamella?, do not 

 usually range above 15° (oligoclase-andesine), and the refractive index is 

 about equal to that of quartz. 



The ferromagnesian minerals, which are very abundant, and appear to 

 have crystallized simultaneously, consist of green hornblende (with occa- 

 sional idiomorphic contours) and brown biotite much altered into chlorite 

 and epidote. 



Sphene is also plentiful in somewhat large irregular crystals ; a few 

 small needles of apatite occur, but iron-ores are very sparingly de- 

 veloped. 



