1921] NATURAL SCIENCES OF PHILADELPHIA 179 



less albite crystals. Minute, transparent, greenish, highly modi- 

 fied beryl crystals occur rarely implanted on the albite, while not 

 infrequently the rest of the cavity is filled with matted masses of 

 dark greenish actinolite needles. Occasionally a peculiar green 

 and white porphyritic rock occurs near the margin of the dikes, 

 especially in the Sylmar district. Examination of thin sections of 

 of this material shows the phenocrysts to be aggregates of zoisite 

 in a groundmass of serpentine, and actinolite needles, (fig. 8). Such 

 an association in which the zoisite was pink in color has been briefly 

 described by Wherry. ^^ 



Adjacent to the albitite dikes are usually three distinct zones of 

 minerals formed by the reaction of the pegmatitic solutions with 

 the serpentine walls. In passing from the albitite to the serpentine 

 their order is: vermiculite, actinolite with more or less talc, and 

 talc — beyond which lies the serpentine (fig. 9). 



The vermiculite zone consists of brown scaly vermiculite, repre- 

 senting a weathered biotite. Analyses of the variety jefferisite 

 from Brinton's quarry, Chester county, are given below. Masses 

 of vermiculite may occur in the albitite. The actinolite zone, 

 which is thinner than others, consists of greenish radiating crystals 

 of actinolite with more or less talc. The talc zone consists of silvery 

 white to light greenish, massive or foliated talc, in which are us- 

 ually disseminated many minute octahedral crystals of magnetite. 



It will be observed that there is a gradual increase of magnesia 

 and water in passing from the albitite to the serpentine. From 

 this it seems probable that the peridotite had been serpentinized 

 prior to the intrusion of the albitite, which had the effect of par- 

 tially dehydrating the serpentine in the vicinity of the intruded 

 mass. In the formation of the biotite (now vermiculite), actinolite, 

 and talc zones, the pegmatitic solutions were more or less depleted 

 of silica, alumina, lime, potash, and fluorine. The potash, fluorine, 

 and alumina went to form the biotite zone, magnesia and perhaps 

 iron being furnished by the serpentine. Most of the silica was used 

 in silicating the serpentine to actinolite and talc, the physico-chemi- 

 cal conditions being such as to cause the formation of hydroxy- 

 silicates. The result was that the original pegmatitic solutions, 

 which under normal circumstances would have crystallized into a 

 rock composed of quartz, microcline, albite or oligoclase, muscovite 



^1 Edgar T. Wherry, Some Minerals from Sylmar, Pennsvlvania, Am. Min., 

 3: 47, 1918. 



