GNEISS. 49 



seliloni present in large quantity, except in those rocks which are chissed 

 distinctly as amphibolites, Biotite is in some cases tlie sole mica, but 

 frequently muscovite is associated with it in subordinate quantity. A careful 

 search with the lens is often necessary to determine the presence of plagio- 

 clase. The feldspars are generally white, but in the Mosquito, Horseshoe, 

 and Twelve-Mile amphitheaters a pink or reddish color predominates. In 

 these cases the pegmatite which forms veins in the schists is also pinkish. 



Microscopic constitution. — A microscopical examination reveals the presence 

 of microcline in small quantities, while ordinary plagioclase is very abundant, 

 as is also muscovite frequently intergrown with the feldspars Apatite and 

 ilmenite are the most common accessory minerals, the latter giving rise 

 to titanite in the form originally called titanomorphite by von Lasjiulx.^ 

 Isolated rounded grains, which are nearly or quite colorless and but very 

 faintly dichroic, are doubtless referable in part to titanite and in part to 

 pyroxene of a variety near sahlite. The dark portion of a banded gneiss 

 from the Arkansas anqjhitheater consists principally of quartz, three feld- 

 spars, biotite, and hornblende. The last two minerals are often intergrown 

 in a peculiar manner, the biotite leaves being parallel to the orthopinacoid 

 of the hornblende. Ilmenite is abundant and passes by alteration into 

 " leucoxene," which appears dull white by reflected light. This again passes 

 into a granular mineral resembling titanite, although not very strongly 

 dichroic. Blood-red films of hematite ai-e discovered in the leaves of biotite. 



In the porphyritic gneiss of the Platte amphitheater microcline is an 

 important element. One large grain of it contains inclusions of quartz and 

 mica in considerable quantity. Muscovite, which is not prominent macro- 

 scopically, is abundant in delicate plates intergrown with tlie feldspars, 

 eitlier parallel to the common crystal faces or without regularity. This 

 muscovite seems to be original and not a decomposition product. An 

 intergrowth of biotite and muscovite, whereby a crystal of the former is 

 surrounded by a zone of the latter having the same orientation, Avas also 

 ol)served. Quartz grains contain biotite cry.stals, needles of rutile (I), and 

 double fluid inclusions with carbonic acid The pink feldspar in gneiss 



'A. vou Lasaulx, Nenes Jalirlmch fiir Min., etc., 1S79, p. 563. 

 MON XII 4 



