1890.] NATURAL SCIENCF:S of PHILADELPHIA. 435 



December 2. 

 Mr. Theodore D. Rand in the chair. 

 Sixty-eight persons present. 



A paper entitled " Description of a new Japanese Scalpelluni," 

 by Henry A. Pilsbry, was presented for publication. 



The death of Samuel Lewis, M. D., a member, was announced. 



Geology of the South (Chester) Valley Hill. — Mr. Theo. D. Rand 

 remarked that in a paper on the serpentines he had called attention 

 to the fact that northwest of West Chester there appeared to be an 

 outcrop of Laurentian rock on the north side of Black Horse Run 

 or Taylor's Run apparently enclosing a mica schist valley. He 

 had expressed doubt as to whether it was really Laurentian. 



A further study of this region convinced him that these gneisses 

 extend several miles at least, and certainly from the east branch of 

 the Brandywine to the High Street Road near Taylor's Mill, and 

 that they belong properly to the mica schist formation of Cream 

 Valley which bounds the serpentine on the north and further that 

 these schists, ofteu garnetiferous, compose the greater mass of this 

 part of the South (Chester) Valley Hill, which has, he believed, 

 been heretofore universally regarded as wholly of the hydromica 

 schist of which it is composed a few miles to the northeast. It is 

 true that in some places it is extremely difficult to tell one rock from 

 the other, but where largely exposed the separation is clear. Small 

 hand specimens often look alike, rock masses do not. 



Where garnets occur they are a distinguishing feature, as he be- 

 lieved they are not found in the true hydromica schist. When gar- 

 nets are absent, the most characteristic feature is the occurrence of 

 the quartz, which, in the hydromica, is in lenticular masses closely 

 covered with the hydromica which adheres strongly, while in the 

 mica schists the quartz is either disseminated, or interlaminated in 

 sheets frequently very thin. The fracture too is characteristic, the 

 hydromica breaks into more or less lenticular masses, the tendency 

 being towards very thin flat plates with very smooth surfaces, or 

 more usually into elongated roughly oval masses on which parallel 

 faces are almost entirely absent but still with an even smooth sur- 

 face. The mica schist on the contrary is more massive, has a more 

 or less regular lamination, breaking into masses with two nearly 

 parallel sides, the other fractures irregular but without the tendency 

 to come to a thin edge prevalent in the hydromica. The surfaces 

 are rougher and somewhat corrugated. The mica schist occasion- 

 ally contains feldspar, the hydromica probably never. 



There is in some places at least, a marked change of dip, that 

 of the hydromicas being almost invariably 80° and upwards and 



