1890.] NATURAL SCIENCES OF PHILADELPHIA. 77 



Mr. Hall regards the rocks along Chester Creek from a mile south 

 of Lenni to Cheyneys station, except a narrow belt at Glen Mills, as 

 Laurentian underlying the schists and exposed by the deeper erosion 

 along the creek, ^ but they are so unlike any of the rocks in the ad- 

 mitted Laurentian belt and bear so close a resemblance to the hard- 

 er gneisses of the Manayunk series that I believe Prof. Rogers' 

 observations to be correct and that the Laurentian is included 

 between gently curved and almost straight lines and that it does not 

 present the contorted outline shown on Mr. Hall's map. These 

 rocks are more fully described in connection with the Glen Mills ser- 

 pentine. In the northwesterly boundary also of the Laurentian 

 my observations confirm those of Prof. Rogers. Mr. HalPs map 

 makes a schist area overlie the Laurentian from Bryn Mawr to 

 Wayne.- In this area, I have described' many outcrops of 

 typical Laurentian, well exposed, ■with not a trace of the schists west 

 of Rosemont except the narrow belt on the northwest of the Lauren- 

 tian along the bottom of Cream Valley. 



To define these areas more exactly will require very close and 

 careful observations of excavations hereafter made, for the covering of 

 soil in many places is so great that only thus can the underlying 

 rocks be known. 



This Laurentian belt is the third of Prof. Rogers. Immediately 

 southeast of it lies his second belt, and southeast of the second his 

 first. Of these he writes, and most accurate are his descriptions :- — 



The First or most southern general division or group, may be approximately 

 defined as extending from the lowest exposures on the river, or those near Gray's 

 Ferry, to the upper end of Manayunk ; the second or middle belt, extending from 

 Manayunk past the serpentine and soapstone range to a line a little north of the 

 upper boundary of the County or City of Philadelphia ; and the third or northern 

 extending thence to the northern edgeof the whole gneiss formation, and it is over- 

 laid or limited by the older primal rocks in the vicinity of Spring Mill. 



First Belt. The southern or Philadelphia belt contains the following chief 

 descriptions of ordinary gneissic rock, with many subvarieties. The most common 

 or typical variety of all is the gray, bluish, rather finely laminated triple mixture of 

 quartz, feldspar and mica, the quartz for the most part, white or transparent ; the feld- 

 spar usually white, and very generally somewhat chalky from incipient decomposi- 

 tion ; and the mica most commonly black or dark brown and in small plates. This 

 rock occasionally includes small insulated garnets. 



The next most common species is a dark bluish-gray, sometimes greenish-black 

 gneiss, composed of hornblende and quartz, with sometimes a little feldspar, the 

 hornblende always greatly predominating. The rock is very usually fine grained 

 and thinly bedded, its fracture and strucmre being controlled by the general paral- 

 lel crystallization of the prevailing hornblende. 



1 2nd Geol. Survey of Pa., C^ p. 2. 



2 Qs 22. 



3 2nd Geol. .Survey of Pa., An. Rep. 1S86, IV, p. 1573, etc. 



