1921] NATURAL SCIENCES OF PHILADELPHIA 177 



As is characteristic of pegmatites in general, the albitite dikes 

 are exceedingly irregular, pinching and swelling, and very frequent- 

 ly bifurcating. Examples of the irregularities are shown in fig. 3 

 and fig. 4. 



The albitites consist almost entirely of granular white, yellowish, 

 or bluish gray, translucent albite." Most of the albite is un- 

 twinned, — a peculiarity noted above as being shown by the albite 

 of the California albitite, and in the oligoclase of the Natal pluma- 

 site. A yellowish-white untwinned albite from Campbell's quarry, 

 two miles northwest of Sylmar gave the following indices of refrac- 

 tion upon immersion, indicating a quite pure albite: a = 1.526, 

 P= 1.530, 7=1.536, all = 0.0005 A granular grayish white al- 

 bite from the vicinity of the Line Pit (probably from the present 

 site of Garrison's quarry, three-quarters of a mile west of Rock 

 Springs) was analyzed by Brush^o with the following results: 



Table 2. Analysis of Albite. 



Granular, grayish-white; near the Lancaster County chrome mines. Analysis 

 by Brush. 



SiOj 66.65 



AI0O3 20.79 



MgO 0.52 



CaO 2.05 



Na,0 9.36 



99.37 

 Sp. gr 2.619 



Quartz is conspicuous by its absence, having been noted at but 

 one locality, a quarry north of Black Run, one and a half miles 

 northwest of Sylmar, where it occurs with much muscovite and 

 anorthoclase (fig. 7). The most frequent accessory minerals are 

 hornblende and biotite. Hornblende forms minute radiating groups 

 while biotite occurs in minute crystals, scattered through the al- 

 bite, at times occuring locally in large amount. Tourmaline is 

 abundant in the albitite of Brinton's quarry, and molybdenite has 

 been found in the quarries northwest of Sylmar. Actinolite some- 

 times occurs in beautiful greenish radiations through the albite, 

 especially near the margins of the dikes. The albitite of Wiant's 

 quarry, three-quarters of a mile northeast of Pilot, Cecil County, 

 is filled with miarolitic cavities which are lined with minute, color- 



" Called "leelite" in some of the earlier lists of Pennsylvania minerals. 

 2" Am. J. Sci. (2), 8: 390, 1849. 



