Granites of Cecil County, in North-eastern Maryland. 103 



land, which Heddle had described under the name of xantho- 

 me. Lacroix showed this mineral to be merely a staurolite 

 whose composition was rendered variable through the pres- 

 ence of more or less mica. This staurolite also contained 

 rutile in crystals of considerable size. Terniier* noted in 

 some staurolites from near St. Etienne (Loire), that the crys- 

 tals were surrounded by a rim of white mica and chlorite 

 which had resulted from their alteration, so that between 

 cross nicols a thin section of the mineral was surrounded by 

 a peripheral zone of bright colors. When the rim of mica 

 was very wide, it was observed that the crystals were much 

 corroded, and that many small fragments of unaltered mate- 

 rial were scattered through the mica, all of which were simi- 

 larly oriented, with the main part of the staurolite crystal. 

 On account of the bright polarization colors of these mica 

 pseudomorphs they have been called "shimmer-aggregates" 

 by Barrow, t in his work on the gneisses in the south-east 

 Highlands of Scotland. In the rocks of that region, this 

 author noted that the replacement of staurolite by mica starts 

 from the outside and then proceeds to the interior along 

 cracks, so as to leave cores of the unaltered material. Bar- 

 roisj noted in the Rostrenen granites an analogous change 

 of andalusite and chiastolite to a fine-grained aggregate of 

 white mica. 



The fullest, as well as the most recent, description of this 

 change is that given by Lacroix|| in his recent book on the 

 mineralogy of France. He found, near Itsatsou, in the 

 Lower Pyrenees, staurolite crystals which had altered to 

 mica. The change was generally peripheral, but it followed 

 the fractures and cleavage lines. The alteration was incom- 

 plete, for a corroded fragment of the staurolite always re- 

 mained at the center. The mica plates reached an extreme 

 length of one millimeter. 



A chemical analysis of this altered staurolite in the Liberty 

 Grove schist was made by Mr. George Steiger, under the direc- 



:: Bull. de la Soc. Min. de France, Paris, Vol. XII, p. 395, 1889. 



t An Intrusion of Muscovite-Bioiite Gneiss, Quat. Journ. Geol Soe.. Vol XLI.X . 

 p. 340, 1893. 



JLe Granite de Rostrenen, Societe Geologique du Xord, Vol. XII, pp. 57, 69, 80, 

 91. 1884. 



Mineralogie de la France et de ses Colonies, p 5, with figure, 1893. 



