ACID IXTEUSIVES. " 193 



fine to medium, as do the biotite-grauites proper. In general they may be 

 described as biotite-grauites in which the micropegmatitic iutergrowth of 

 quartz and feldspar instead of being subordinate preponderates. Many of 

 the Avell-crystallized feldspars are surrounded by a border of micropegma- 

 tite, which varies from a narrow strip to a very wide border, usually in 

 inverse ratio to the size of the feldspar nucleus. The feldspar in the inter- 

 growth is continuous with that of the nucleus. A coarsely radial arrange- 

 ment of the micropegmatitic iutergrowth was frequently observed. Where 

 the feldspars and quartz are predominantly porphyritic, and micropegmatite 

 forms the groundmass, the rock grades over into the rhyolite-porphyries with 

 micropegmatitic groundmass — the inapj^ropriately named granophyres of 

 Rosenbusch. 



The biotite of the micropegmatitic granites has partly altered to 

 chlorite and sagenite. In some of these rocks the biotite is collected into 

 large aggregates of imperfect individuals, which surround large pieces of 

 iron ore. In some instances it is included in the plagioclase. The biotite 

 flakes in the feldspar are sometimes so numerous as to conceal almost com- 

 pletely the feldspar substance. In one instance the feldspar of such a 

 micropegmatitic iutergrowth is completely replaced by biotite. These sec- 

 ondary biotite flakes surrounding the remaining more or less rounded quartz 

 areas of the micropegmatite produce a rock which is strikingly like a mica- 

 schist in places, although it is of unquestionably eruptive character. 



Tourmaline is a rare accessory in these granites, a small smoke-brown 

 crystal having been observed in one section. Titaniferous iron ore altering 

 to leucoxene or sphene and rutile is found, as are also the common accessory 

 minerals — apatite, rutile, and zircon. They contain also the same secondary 

 minerals as the normal biotite-granite. 



MUSCOVITE-BIOTITE-GRANITE. 



These are medium-grained rocks, and, owing to the fact that the mus- 

 covite is more abundant than the biotite, have a light-gray color. 



The muscovite is noticeably automorphic with respect to the biotite, 

 though the biotite is also in weH-developed automorphic plates. Plagio- 

 clase is present in these granites in very large quantity. It shows an excel- 

 lent zonal development, with diminishing angle of extinction — that is, 

 increasing acidity — from the center outward. The center of the individuals 



MON XXXVI 13 



