406  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull.  86. 
from  certain  forms  of  the  more  micaceous  beds  of  the  true  gneissic  or 
Hypozoic.  It  is  impossible  to  subdivide  the  members  of  the  Lower 
Primal  group  in  southern  Pennsylvania,  because  of  a  prevailing  trans- 
verse cleavage,  which  extensively  effaces  all  clear  traces  of  the  original 
bedding,  and  because  of  the  presence  of  innumerable  plications,  often 
so  closely  compressed  as  to  appear  as  only  one  uniform  dip,  the  anti- 
clinal and  synclinal  foldings  in  many  cases  escaping  detection  through 
the  obscuring  influence  of  cleavage,  and  because  of  mutations  in  the 
composition  of  the  beds.  The  rocks  between  the  Primal  white  sand- 
stone and  the  genuine  gneiss  then  include  talcoid  siliceous  slate, 
talco-micaceous  slate,  and  schistose  and  quartzose  micaceous  rock.  On 
the  Brandy  wine  the  massive  gneisses  and  finely  laminated  material  are 
interlaminated  in  such  a  way  as  to  lead  to  the  conclusion  that  the  lat- 
ter are  closely  infolded  in  the  older  metamorphic  series.  In  the  Primal 
of  Susquehanna  and  York  counties,  the  true  bedding  is  very  obscure, 
being  almost  obliterated. 
In  passing  southward  on  the  Susquehanna  the  rocks  become  steadily 
more  crystalline,  until  they  are  so  altered  as  to  have  been  hitherto 
mistaken  for  the  true  Hypozoic.  The  precise  line  of  contact  of  the  lime- 
stones with  the  slates  is  not  clearly  visible  at  times ;  indeed,  there 
seems  to  be  no  line  of  sudden  transition.  The  cleavage  planes  are  in 
general  parallel  with  those  of  the  original  bedding.  The  dips  on  this 
river  are  steadily  iira  southeast  direction  for  a  distance  of  7  or  7£  miles, 
and  it  is  believed  that  the  rocks  consist  of  many  compressed  folds  which 
repeat  the  same  strata  many  times  over.  Southwest  of  the  Susquehanna, 
in  the  South  mountains,  in  Adams,  Cumberland,  and  Franklin  counties, 
is  an  extensive  area  which  is  placed  with  the  Primal  series.  It  is  a 
continuation  of  the  Blue  ridge  of  Maryland  and  Virginia.  There  are  a 
few  intrusive  rocks,  mostly  of  greenstone  and  trap.  Some  of  the  rocks 
are  very  crystalline,  but  none  are  regarded  as  belonging  to  the  gneissic 
series.     In  this  series  are  found  limestones  associated  with  iron  ore. 
Leeds,149  in  1870,  states  that  on  the  Germantown  railroad,  3  miles 
from  Philadelphia,  in  the  micaceous  schists  are  imbedded  huge  bowl- 
ders of  hard,  compact  hornblende  rock.  They  are  supposed  to  be  a 
primitive  surface  formation  which  was  broken  up  before  the  deposition 
of  the  metamorphic  rocks  of  undetermined  age. 
Frazer,150  in  1876,  describes  several  sections  in  York  and  Adams 
counties.  Here  are  included  hydromica- slates  and  hydromica-schists, 
chloritic  rocks,  quartzite,  quartz-slate,  gneissoid  mica-schist,  limestone, 
and  chert.  Several  sections  show  an  unconformable  contact  between 
the  York  limestone  and  the  crystalline  schists.  The  latter  usually 
dip  at  a  high  angle. 
Frazer,151  in  1877,  describes  cross- sections  in  the  counties  of  York, 
Adams,  Cumberland,  and  Franklin.  In  South  mountain  the  structure 
is  found  to  be  essentially  the  same  as  that  given  by  Eogers,  except 
that  it  also  contains  limestone.    In  one  section  is  a  thickness  of  over 
