388 THE CRYSTAL PALLS IRON-BEARING DISTRICT. 



an axis, shows a parallel arrangement of the micas on all sides, and a con- 

 tinuous glisten follows the revolution; while on a surface at right angles to 

 this direction the micas are not parallel and wind about the other constitu- 

 ents indifferently. In the more fissile varieties the outcrojjs often have a 

 rough, channeled surface, suggestive of the surfaces familiar in closely 

 crenulated mica-schists, or on the corrugated walls of a fault. Similar cor- 

 rugated surfaces frequently part more massive from more fissile parts of the 

 same outcrop. 



Under the microscope the essential constituents of the granites and 

 granitic gneiss are seen to be quartz, orthoclase, microcline, plagioclase, 

 biotite, and muscovite, with the iron ores, titanite, and occasionally apatite 

 and zircon as accessories. In the massive phases the general relations of 

 these minerals to one another, and their order of crystallization, in no respect 

 differ from those of igneous granite. The quartz, which is the last mineral 

 to form, contains numerous fluid and gas inclusions, the former often with 

 a moving bubble. Of the feldspars microcline is much the most common, 

 then plagioclase, while orthoclase is generally comparatively rare, although 

 sometimes it is more abundant than the microcline. The plagioclase, from 

 its relief and extinction angles, is probably not lower in the scale than oligo- 

 clase. The orthoclase is usually clouded with alteration products, and some- 

 times the dull interior is surrounded with a narrow unattacked rim. Both 

 micas are always present as original minerals, and on the whole biotite is 

 the more abundant. They occur in small stout crystals, often as inclusions 

 in the quartz and feldspars. Magnetite is rare, but occurs in idiomorphic 

 forms in the later constituents, as do also minute crystals of zircon and 

 apatite. Thin sections of even the most massive-looking specimens invari- 

 ably show the effects of pressure in the undulatory extinction of the quartz 

 and in the bending and occasional fracture of the feldspar. 



In the foliated varieties with which these massive varieties are closely 

 associated the effects of mechanical stresses are the striking microscopic 

 phenomena. The constituent minerals are essentially the same as in the 

 massive phases, but the micas are relatively more abundant. The quartz 

 and feldspar individuals are fractured and strained, and occur in irregular 

 cores separated by anastomosing zones of a fine quartz-feldspar mosaic. In 

 these last, new micas, in long curving individuals and clusters, have been 

 developed in great numbers. 



