1890.] NATURAL SCIENCES OF PHILADELPHIA. 79 



more schistose class of the gneissic rocks. This is a gray fine grained binary 

 mixture of granular quartz and minutely crystallized scales of mica, the quartz 

 being the prevailing element. It is a species of whetstone and some of the more 

 quaitzose bands would furnish masses well suited for employment as whetstones 

 for scytlies. A very common, indeed a characteristic of this quite remarkable 

 and extensive division of the micaceous gneiss group, consists in its peculiar 

 fracture; the rocks breaking into long narrow chunks, comparatively smooth on 

 their sides, but excessively ragged on their ends; a style of fracture strongly 

 resembling that of half rotted fibrous wood. This peculiar rock is in greatest 

 force toward the northern side of the middle gneiss belt, or between the serpen- 

 tine or steatite, and the hard feldspathic gneiss of the southern margin of the 

 third or northern gneissic belt. It is interstratified toward its southern side, with 

 more or less frequent and thick bodies of the other variety of mica slate possessing 

 mica in large and twisted scales, with an abundance of garnets. On its northern 

 side it alternates to some extent with a greenish talcose slate, or, what is the same 

 thing, the talc in this quarter replaces the mica in whole or in part in certain 

 divisions of the group." 



There is probably in the rock termed by Rogers " altered primal" 

 (whatever may be its geological age) a key rock, for this seems to 

 lie quite persistently along both edges of the Laurentian though only 

 occasionally visible. West of the Schuylkill it is well exposed, but 

 it may be seen also at Willistown and near Westtown, Chester Co. 

 It is thus described by Prof. Rogers, p. 154 : — 



" In the district of the Schuylkill and Wissahickon, the three members of which 

 the primal series there consist, present the following aspects and dimensions : 



The lowest or semi-porphyroidal group, evidently an altered sandy-slate or ar- 

 gillaceous sandstone, is remarkable for the regular parallelism of its lamination and 

 bedding; the laminaj alternately light and dark, being exceedingly thin, many of 

 them usually packing within the thickness of an inch. These laminae consist, 

 where the rock wears its most metamorphosed form, of white earthy imperfectly 

 developed feldspar, and perfectly developed hornblende. Besides these alternate 

 whitish and dark streaks, the cross fracture of the rock displays a multitude ofovoi- 

 dal concretionary crystallizations, generally only specks in size, but sometimes of 

 the dimension of bullets, the larger and better formed concretions being frequently 

 genuine crystals of feldspar. In some of the layers certain laminae are studded 

 with isolated crystallizations of hornblende." 1st Geological Survey of Pennsyl- 

 vania, Vol. 1, Page 68. 



Mr. Hall divides Rogers' first and second groups into three, in- 

 serting a belt (the Manayunk schists) between Rogers' first and 

 second ^ and states that it contains " alternations of the varieties 

 of gneiss characteristic of the first belt and a predominance of 

 sandy gneiss composed of quartz and a small amount of feldspar 

 and mica in minute flakes. Mica schists and hornblendic slate 

 alternate with finer grained gneisses, the mica usually light colored." 

 He further states that it includes part of Rogers' second belt. 



It is possible that this arises from a misunderstanding of Prof, 

 Rogers' use of the term " upper end of Manayunk," which is ev- 

 idently used loosely. Rogers says " approximately defined." In- 



1 2nd Geol. Surv. Penna. C p. 2. 



