1921] NATURAL SCIENCES OF PHILADELPHIA 183 



Unionville, Chester County.^^ TheUnionville corundum mines, 

 now abandoned, are situated in Newlin township, respectively one 

 and a half antl two miles northeast of Unionville (fig. 2), at the con- 

 tact of serpentine and mica gneiss. Many boulders of corundum 

 were obtained in the fields by the farmers who systematically 

 dropped a crow-bar in the soil in the trail of the plow. 



At the pits one and a half miles northeast of Unionville the cor- 

 undum was found in a rock consisting of granular white albite, cor- 

 undum, black tourmaline, margarite, and euphyllite — a basic so- 

 dium and potassium mica. The corundum occurred in white or 

 grayish crystals. Zoisite was abundant, and occasionally black 

 octahedra of spinel were obtained. Adjacent to the vein the fol- 

 lowing minerals were noted: talc in white foliated masses and green- 

 ish actinolite with much dark green chlorite. 



At the mine one-half mile north of the preceding one a vein of 

 corundum and margarite seven feet thick was uncovered, which 

 was separated from the serpentine by a zone of foliated talc. In 

 this a vein of fawn-colored cliaspore measuring three by two feet in 

 length and width, and two inches in thickness was discovered, some 

 of the crystals of which were two inches long.^^ The corundum 

 was white, brownish gray, to bluish. Crystals not infrequently 

 were sheathed with margarite or damourite. Margarite formed 

 in yellowish or pinkish foliated masses, or secondary aggregates of 

 scales with cores of corundum. Associated with the margarite 

 were honey-yellow or fawn colored laminated masses of diaspore, 

 occasionally in perfect spendent crystals up to two inches in length. 

 Beautiful translucent bluish or greenish prismatic crystals of tour- 

 maUne penetrate the diaspore. The association is such as to in- 

 dicate the order of crystallization: tourmaline, diaspore, margarite. 

 Jefferisite (vermiculite) is found in large yellowish green plates. 



The plagioclase at the Unionville mines varies from white albite 

 to yellowish striated oligoclase. Analyses of these are given be- 

 low. 



^' H. D. Rogers, Geology of Pennsylvania, I, 1858, 170. Joseph Willcox, Sec. 

 Geol. Surv. Penna. Rep., C4: 346, 1883. Joseph Leidy, Proc. Acad. Nat. Sci. 

 Phila., 1872: 238-239. Frederick A. Genth, Proc. Am. Phil. Soc, 13: 361-406, 

 1873; 20: 381-404, 1882. The statement "about one mile from Unionville on 

 the Kennett Square road" means from the upper mine, and not the village of 

 Unionville. 



34 Isaac Lea, Proc. Acad. Nat. Sci. Phila., 19: 44-45, 1867. 



