80 PROCEEDINGS OF THE ACADEMY OF [1890. 



asmuch as just above Mauayunk the more massive and less garnet- 

 iferous schists of the first belt are well exposed, while about two 

 miles above the centre of Manayunk the rocks of his second belt 

 occur in most characteristic form, is it not most probable Prof. 

 Rogers intended to refer to this point, especially, as when he wrote 

 there was no well known place nearer than Manayunk? INIr. Hall's 

 term, however, is convenient, but I regard it, and use it in these 

 notes, rather as a synonym for all of Rogers' first belt except the 

 Fairmount gneiss, and the porphyritic gneiss. 



By far the best exposure of these rocks as described is along the 

 Schuylkill, but the structure is far from clear. The Fairmount 

 gneiss, identical with the gneiss at the Chester, Leiperville and 

 Avondale quarries is generally supposed to underlie the more mica- 

 ceous rocks to the northwestward, yet almost everywhere is a pretty 

 uniform northerly dip. There are alternations of soft highly mica- 

 ceous schists passing by an increase of quartz and feldspar and 

 decrease of mica into gneissoid schists and gneiss, also hornblende 

 schists and gneisses, but in no case, except perhaps northwest and 

 southeast of the porphyritic gneiss can a repetition be found to 

 prove the apparent monoclinal to be, as it has been supposed to be, 

 a succession of compressed synclinals and anticlinals. 



To the southeastward, the rocks as a rule are highly micaceous, 

 and in these the Faiimount gneiss appears to rise as an anticlinal. 

 In the cut of the Pennsylvania Railroad, about 33rd Street, thesouth- 

 easterl}' dip at a moderate angle was clearly shown ; the northwest- 

 erly edge has not been uncovered, but the strata weie nearly hori- 

 zontal and were succeeded by mica schist within a hundred yards. 



The mica schists are visible southeast of the Fairmount gneiss, with 

 gentle undulating dips, on the west side of the Pennsylvania Railroad 

 near Powelton Avenue, and on both sides, at the approach to the 

 tunnel at 31st and Market Streets ; northwestward of the Fairmount 

 gneiss are alternations of mica schists, micaceous gneiss, hornblendic 

 gneiss and allied rocks to the Falls of Schuylkill. 



On the Schuylkill a short distance west of the Girard Avenue 

 bridge there was exposed a rock, mica schist on the outside, horn- 

 blende schist within, and the passage of the one into the other was 

 evident. At the quarry (now a coal yard) on the Germantown road 

 at the crossing of the Germantown branch of the Reading Railroad, 

 a similar change was apparent, the micaceous mineral here being the 

 Philadelphite. INIay not these occurrences throw light upon the fact 



