1890.] NATURAL BCIENCi:S OF rillLADELPIIIA. 115 



I think the weight of the evidence is, that underlying this region 

 are highly feldspathic schists inclosing beds of enstatite and perhaps 

 other hornblende-like minerals, that through this probably penetrate 

 dykes of granulite, that the enstatite, etc. have become serpentine 

 and allied minerals, while the schists and granulite have decomposed. 

 The serpentine also in many parts has changed into quartz, and this 

 quartz now lies strewn upon the surface as the most abundant rock, 

 simply by reason of its stability. Were a section of rock from the 

 Glen Riddle cut spread upon land and exposed to the weather for a 

 few seasons, probably nothing would be left but soil and the honey- 

 comb quartz, which last would probably appear to be the chief, if 

 not the only rock, while in fact it does not constitute two per cent of 

 the volume. This theory would account for the excellence of the 

 soil in most parts of this area, indeed over nearly all of it where the 

 serpentine is not actually visible at the surface. This was well de- 

 scribed by an intelligent farmer with whom I conversed, in the 

 words, " where the serpentine lies deep the ground is good." I am 

 informed that in nearly all the wells, even where the honey-comb 

 quartz is quite abundant at the surface, the rock reached is a gneiss 

 or schist. 



In one respect this Media outcrop is anomalous. It appears to be 

 the only one away from the Laurentian in which enstatite appears. 

 On the east branch of Ridley creek, that is northeast of the table- 

 land, there is a small outcrop of this rock with a strike which would 

 carry it into the area under consideration. This enstatite is in the 

 schistose gneisses, apparently as an interbedded mass, and is in the 

 direct line of the outcrops arranged in line to Palmer's Mills as 

 shown on Hall's map. The exposures are poor, but that they are 

 all within the schists is unquestionable and with them in some cases 

 a coarse granite is associated, apparently interstratified in the schists. 



It is further true that the northwestwardly part of the Media out- 

 cro}), or rather series of outcrops, does border the Laurentian, and 

 that the serpentine lying along Dismal Run may be a continuation 

 of the Blue Hill serpentine, and may be entirely distinct from the 

 southerly outcrops which are certainly in the schists. 



The Serpentine of Glen Mills on Chester Creek. 



Mr. Hall finds a narrow synclinal of schists with serpentine over- 

 lying the gneisses of Chester creek which he regards as Laurentian. 

 I have already stated my reasons for thinking these gneisses a part 



