106 PROCEEDINGS OF THE ACADEMY OF [1890. 



from the large INIarple outcrop south of Newtown Square, most of 

 them in two lines the continuation of which marks the northwestern 

 and southeastern margins of the many adjacent outcroj^s west of 

 INIedia. He also maps several south and southwest of the latter. It 

 is not an easy task to map these serpentine areas, their margins are 

 ill defined, and generally covered with soil. While, therefore, their 

 general location may be correct, in some minor points they do 

 not agree with my observations, but accurate mapping would require 

 a topographical survey and numerous excavations. 



I have visited most but not all of the minor outcrops. At 

 some I was unable to find the serpentine, but wherever found it was 

 apparently a small bed of hornblende-like rock interst ratified in the 

 Manayunk gneisses, altered more or less into antholite, steatite, ser- 

 pentine and honeycomb quartz, except the outcrop on the east bank 

 of Ridley creek northwest of Media where there is unaltered ensta- 

 tite, also apparently a narrow interstratified bed. 



On Mr. Hall's ma}) a small area of serpentine is shown as almost 

 a continuation of the great Marple outcrop and both are represented 

 to be in the schists. I believe the Laurentian to bound the 

 Marple serpentine on the northwest as it certainly does a short 

 distance to the northward. Between the two areas intervene Chest- 

 nut Hill schists with the chai-acteristic quartz in large quantity. 

 The rocks of the small area are steatite and antholite and very un- 

 like the enstatite and hard black serpentine of the large outcrop. 



At this locality and southwestward many of the outcrops of ser- 

 pentine are accompanied by outcrops of?, granite, or granulite, often 

 coarse, sometimes fine grained, containing orthoclase, oligoclase, 

 quartz and mica, the latter in small quantity but at times in large 

 ■crystals, the quartz also in small quantity, the mica being rarely dis- 

 seminated. This will be more fully described in the consideration of 

 the Media outcrop. 



The Larue Outcrop West of Media. 



This outcrop, or rather series of outcro]is, occurs in or on the 

 borders of an elevated table-land, occupying the greater part 

 of the area lying between Ridley and Chester creeks, extending 

 southwardly a little south of the railroad from Philadelphia via 

 Media to West Chester. On the northwest, the road from Lenni, 

 through Lima, to Sycamore Mills is on it, but near the edge, while 

 on the north. Dismal Run marks the boundary and on the east. 



