182 PEOCEEDINGS OF THE ACADEMY OF [Part I 



PENNSYLVANIA PLUMASITES.^" 



Plumasites or corundum-bearing pegmatites occur in Pennsyl- 

 vania at the following localities: Morgan Station and Black Horse 

 in Delaware County, (fig. 10), and near Unionville, Chester 

 County (fig. 2) . All form intrusive masses in serpentine, or at the 

 contact of serpentine and gneiss. 



The geology of the region has been described by Rogers," Frazer 

 and Hall, 2^ Rand,^^ Bascom'" and others. The area is underlain 

 by Precambrian gneisses and schists into which have been intruded 

 bodies of granite, gabbro, peridotite and pyroxenite. The basal or 

 Baltimore gneiss is a banded quartz-feldspar rock with massive 

 granitic facies. Overlying this is the Wissahickon mica gneiss, a 

 schistose quartz-feldspar rock with an excess of biotite or muscovite. 

 The peridotites and pyroxenites, which are genetically related to 

 the gabbros, in most cases have been hydrated to serpentine. 



Morgan Station (Village Green.) This locality is situated on a 

 very small patch of serpentine intrusive in Wissahickon gneiss one- 

 quarter mile south of Morgan Station, Aston township (fig. 10). 

 Large bronzy brown corundum crystals were found sheathed with 

 white pearly margarite." 



Black Horse, Middletown Township. An old corundum mine 

 was formerly situated in a large area of serpentine about one-quarter 

 mile south of Black Horse, on the west side of the road to Elwyn 

 (fig. 10). Many loose crystals of corundum were found in the soil 

 in the vicinity of the pit, and elsewhere in the township. One half 

 mile to the west are exposed granite gneisses. The corundum in 

 slender bipyramidal crystals, of a white to brown color, often as- 

 teriated,^^ was found in a granular white albite-oligoclase. 



28 As it has been stated that corundum was first found in America in Laurens 

 County, S. C, in 1819, it is desirable to point out an earlier discovery of this 

 mineral at Chestnut Hill, Philadelphia: Adam Seybert, Adamantine spar and 

 basaltes. Medical Repository, 3: 202, 1800; also J. C. Delametherie, J. de Phys. 

 (53), 9: 404-405, 1801. 



2' Henry D. Rogers, Geology of Pennsylvania, 1858. 



2* Second Geological Survey of Penna., Reports C4 and C5, 1883 and 1885. 



s^Proc. Acad. Nat. Sci., Phila., 1900: 160-338. 



3" Bull. Geol. Soc. America, 16: 289-328, 1905; U. S. Geol. Surv. FoHo 162, 

 1909. 



31 Am. J. Sci. (2), 379-380, 1849. 



s^Proc. Am. Phil. Soc, 13: 361-406, 1873; Ibid., 20: 381-404, 1882; Joseph 

 Willcox, Second Geol. Surv Penna., Rep. C4: 346-354, 1883. 



