662  PRE-CAMBRIAN    GEOLOGY   OF    NORTH    AMERICA. 
The  Baltimore  gneiss  is  therefore  held  to  be  pre-Cambrian  in  age 
and  is  correlated  with  the  pre-Cambrian  Stamford  gneiss  of  New 
England,  with  the  Fordham  gneiss  of  New  York,  the  Baltimore 
gneiss  of  Maryland,  and  the  Carolina  gneiss  of  Virginia.  While  the 
pre-Cambrian  gneiss  of  Pennsylvania  is  not  stratigraphically  contin- 
uous at  the  surface  with  the  Baltimore  gneiss  of  Maryland,  the  like 
lithologic  character,  stratigraphic  relations,  and  the  proximity  of 
the  formations  have  found  recognition  in  a  common  name.  The  Bal- 
timore gneiss  embraces  H.  D.  Rogers's  Primal  Lower  Slate  and  part 
of  his  third  gneiss  belt  ("  Hypozoic  ")  ;  the  major  part  of  this  gneiss 
belt  is  gabbro.  Both  the  gabbro  and  the  Baltimore  gneiss  are  in- 
cluded under  the  term  "  Laurentian  gneiss  "  by  the  Second  Geological 
Survey  of  Pennsylvania. 
Wissahickon  mica  gneiss. — This  formation,  while  showing  great 
local  variation,  may  be  described  as  a  medium-grained  quartz-feld- 
spar rock  characterized  by  an  excess  of  biotite.  The  gneissic  structure 
is  due  to  beds  of  varying  composition,  quartzose  or  feldspathic  beds 
alternating  with  excessively  micaceous  layers.  Hornblende,  garnets, 
tourmaline,  andalusite,  sillimanite,  and  zoisite  are  accessory  meta- 
morphic  minerals. 
The  mica  gneiss,  with  its  associated  igneous  intrusives,  extends 
from  New  Jersey  to  Maryland  and  from  Buck  Ridge,  south  of 
Chester  Valley,  eastward  to  Delaware  River,  where  it  passes  under 
a  cover  of  Cenozoic  materials.  It  is  well  exposed  in  the  gorge  of 
Wissahickon  Creek,  from  which  the  formation  takes  its  name.  The 
beds  show  crumpling  and  anticlinal  and  synclinal  folding.  The 
thickness  is  probably  between  1,000  and  2,000  feet. 
The  age  of  the  Wissahickon  mica  gneiss  is  still  in  question;  its 
determination  rests  wholly  upon  evidence  afforded  by  the  strati- 
graphic  relations  which  the  mica  gneiss  sustains,  or  appears  to  sus- 
tain, to  fossil-bearing  sediments.  The  formation  contains  within 
itself  no  clue  to  its  age.  While  the  Wissahickon  mica  gneiss  appar- 
ently overlies  with  anticlinal  structure  presumably  Cambro-Ordo- 
vician  limestone,  that  the  actual  structure  is  one  of  overturned  or 
in  some  cases  fan-shaped  synclines,  bringing  the  mica  gneiss  below 
the  limestone,  is  shown  to  be  presumptively  the  case  by  the  follow- 
ing field  evidence:  The  separation  of  the  Wissahickon  gneiss  from 
adjacent  known  Ordovician  mica  schist;  the  occurrence  in  some 
cases  between  the  gneiss  and  the  limestone  of  tourmaline-bearing 
quartzite;  the  occurrence  within  the  gneiss  areas  of  lenses  of  similar 
quartzite,  and  of  mica  schist  similar  to  that  which  is  interbedded  with 
recognized  Cambrian  quartzite;  and,  finally,  unconformity  between 
the  Wissahickon  gneiss  and  recognized  Paleozoics.  This  evidence  is 
far  from  being  conclusive,  and  present  knowledge  only  estab- 
lishes  a   presumption   that   the   Wissahickon   mica   gneiss   is   prob- 
