1896.] natural sciences of philadelphia. 219 



April 7. 

 Mr. Theodore D. Rand in the Chair. 

 Twenty-five persons present. 



The Serpentines of Eastern Pennsylvania. — Theodore D. Rand 

 called attention to the specimens of serpentine presented this even- 

 ing. They had been collected from numerous localities in south- 

 eastern Pennsylvania. He regarded them, as stated in a paper read 

 before the Academy, as belonging to at least two groups : one bor- 

 dering the ancient gneiss ; the other, which he believed to be much 

 more recent, occurring in the mica schists and gneisses. 



The former are altered igneous rocks, either pyroxenic or chryso- 

 litic, the chief material being enstatite, found often but slightly al- 

 tered ; the latter of more doubtful and perhaps varied origin, deter- 

 mination of which will require much more study of thin sections 

 under the microscope. 



The bright yellow serpentine from Easttown Township, Chester 

 Co., is probably altered chrysolite chiefly, while that from Fritz 

 Island, near Reading, is an altered dolomite. That from Brinton's 

 Quarry; near West Chester, contains brouzite, not entirely changed. 



The Radnor serpentine is chiefly altered enstatite, but specimens 

 presented show, also, a change from asbestus into serpentine. 



No rock is better suited than serpentine to show that minerals 

 have a life history, that they are not the unchangeable substances 

 commonly supposed, for serpentine seems to be a stage in the life of 

 many minerals of which magnesia is a large component, while ser- 

 pentine, in its turn, decomposes into soil, or occasionally, indeed in 

 this region frequently, into quartz. 



Perido- Steatite and Diabase. — Dr. Florence Bascom stated that 

 she had recently made examination of thin sections from the ser- 

 pentine of the belt running northeast and southwest from Chestnut 

 Hill through the soapstone quarry to a point northeast of Bryn 

 Mawr, and also of the trap of the Conshohocken dyke. 



The serpentine was from the quarries on the Black Rock road, 

 between Mill Creek and the Roberts road. The belt lies wholly 

 ■within the mica schists on the southeast side of the Pre-Carabrian 

 gneiss. The serpentine proved to be derived from a peridotite and 

 not from a dolomite or from an enstatite rock, as in other cases 

 mentioned. The thin sections show olivine grains with the charac- 

 teristic alteration to serpentine on their peripheries ; much talc or 

 steatite is present. The rock is, therefore, a perido-steatite. The 

 dark green crvstals, conspicuous in the hand specimens, often 

 twinned, are pseudomorphs after olivine, and not after staurolite, 

 the forms of each resembling the other closelv. 



