102 



Cincinnati Society of Natural History. 



ing grains of varying size, which fill in the space between the 

 mica bands. This mineral contains small dust-like inclusions, 

 also magnetite and chlorite. 



In the portion of the schist exposed in the valley of Basin 

 Run staurolite crystals are very abundant. When these crystals 

 are studied under the microscope they are seen to be completely 

 changed into an aggregate of brightly-polarizing white mica 

 scales, which are associated with chlorite and quartz grains. 

 The quartz in the altered staurolite represents the original in- 

 clusions of this mineral in the staurolite, while the chlorite 

 which is scattered through the altered crystals has wandered 

 from decomposed garnets. No trace of the original stauro- 

 lite remains, even in sections cut through the center of the 

 crystals. The iron hydroxide resulting from this alteration 

 of the staurolite has stained the mica, and outlined small 

 areas of muscovite, which often have a radial arrangement. 

 In some of the less altered crystals faint traces of a former 

 brachypinacoidal cleavage were observed, but even this is 

 obscure. 



Staurolite has generally been considered as a very stable 

 compound, and hence references to its alteration are compar- 

 atively rare in the literature. There are several mentions of 

 its change to steatite, chlorite, and serpentine, but they are, 

 probably, for the most part, erroneous.* The statement of 

 Rand as to the alteration of staurolite to serpentine is cer- 

 tainly a mistake, as this mineral has recently been shown to 

 be not staurolite but olivine. 



The natural course of the alteration of staurolite seems to 

 be to muscovite, and all well-authenticated descriptions which 

 I have been able to find point to this conclusion. In 1871, 

 Pichlerf seems to have been the first to note this alteration. 

 Thiirach,J in his studies on zircon, rutile, and associated min- 

 erals, found staurolite altered to white mica. Lacroix,|| in 

 1886, made a careful study of a mineral from Milltown, Scot- 



*Roth : Allgemeine und Chemische Geologie, Vol. I, p. 384; E. S. Dana, System 

 of Mineralogy, p. 560, 1S92 ; H. Roseubusch, Mikros. Physiog. Mineralien, Vol. I, p. 

 447, 1892. 



t Neues Jahrbueh fur Mineralogie, etc., p. 55, 1871. 



+ Ueber das Vorkommen Mikroskopischer Zirkone, und Titan-Mineralien in 

 den Gesteinen, p. 45, 1884. 



I! Bull, de la Soe. Min. de France, Paris, Vol. IX, p. 78, 1886. 



