PRE-CAMBRIAN ROCKS 



295 



ing of the mineral constituents, are microscopic evidences of an aqueous 

 origin. In the field the gneiss has the appearance of a stratified forma- 

 tion. In the Schuylkill section the formation is heavily bedded and 

 shows fan folding, with crumpling on the periphery (see plates 48 and 

 49). In the center the folds are gentle and open. The formation also 

 contains considerable disseminated graphite, which has been mined in 

 Montgomery and Chester counties. A chemical analysis of assorted 

 samples of the formation shows an acid rock whose composition, as is 

 to be expected in the case of assorted samples, is of a granitic character 

 and not decisively that of a sediment. The two analyses, which repre- 

 sent each a single locality, more distinctly indicate the sedimentary 

 character of the formation. 



Chemical analyses of Baltimore gneiss 



Si0 2 . . . 

 A1 2 3 .. 

 Fe 2 3 .. 

 FeO... 

 MgO... 

 CaO... 

 Na 2 . . 

 K 2 0... 

 H 2 0+. 

 H 2 — 

 C0 2 .... 

 Ti0 2 ... 

 ZrO a . . . 



PA 



CI Not estimated 



F Not estimated 



S 0.09 



NiO Faint trace. 



MnO 0.11 



BaO ... 0.09 



SrO Trace. 



Li 2 None. 



I. 



II. 



III. 



70.21 



72.99 



63.93 



13.95 



10.90 



12.02 



1.05 



0.55 



2.40 



3.08 



2.50 



2.95 



1.26 



1.07 



2.44 



3.10 



1.88 



4.00 



3.27 ' 



3.34 



3.15 



2.69 



1.20 



0.84 



0.48 \ 

 0.19 / 



1.47 



1.30 



0.11 c. 



1.13 



Not det 



0.52 



0.84 



1.04 



Trace. 









-Co 2 



0-51 



0.10 



0.18 



. Not det 



FeS 2 . 



1.6 



Fe 7 S 8 



4.04 



Trace. 



Trace. 



CnO Trace. 



100.30 



99.66 



98.62 



Analysis I was made by W. F. Hillebrand, of the United States Geological Sur- 

 vey, from six samples representing six localities. 



Analyses II and III were made by F. A. Genth, Jr., volume C 6, Second Geolog- 

 ical Survey of Pennsylvania, pp. 116, 105. Material of II was from Johnson's 

 farm, 1 mile east of Feasterville and north of road to Somerton. Material of III 

 was from west side of Neshaminy creek, on the property of Phineas Paxon. 



