656  PRE-CAMBRIAN    GEOLOGY   OF    NORTH    AMERICA. 
Primitive  surface  formation  which  was  broken  up  before  the  deposi- 
tion of  the  metamorphic  rocks  of  undetermined  age. 
Frazer,5  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,6  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  Rogers,  except 
that  it  also  contains  limestone.  In  one  section  is  a  thickness  of  more 
than  17,000  feet  of  quartzite  and  sandy  shale  and  about  2,000  feet  of 
chloritic  slates.  In  another  section  the  rocks  observed  are  quartz 
conglomerate  schist,  jaspery  quartzites,  crystalline  schists,  and  ortho- 
felsites.  The  relations  seem  to  show  an  unconformity  between  the 
older  (Huronian?)  orthofelsites  and  schists  and  the  more  recent 
(Cambrian?)  sandstone,  but  they  would  seem  additionally  to  imply 
that  the  alignment  of  the  one  system  was  the  result  of  causes  entirely 
different  from  and  anterior  to  those  that  formed  the  other.  In 
another  section  the  rocks  increase  in  felsitic  character  to  the  south- 
east and  in  conglomeratic  schistose  character  to  the  northwest.  It 
is  concluded  that  the  South  Mountain  chain  is  composed  of  two 
groups  of  rocks,  the  lower  consisting  of  quartz  conglomerates  in 
which  quartzite  occurs;  the  upper  felsitic  in  character,  containing 
hydromica  schists  and  chlorite  schists.  The  felsite  itself  ranges  from 
a  sandy  slate  to  a  coarsely  porphyritic  rock. 
Hunt,7  in  1877,  states  that  near  Conshohocken  is  a  belt  of  Lauren- 
tian  gneiss,  identical  with  that  of  the  South  and  Welsh  mountains, 
that  separates  the  Philadelphia  gneisses  and  mica  schists,  which  are 
Montalban,  from  the  Auroral  limestone.  The  Laurentian  gneiss  is 
succeeded  on  the  northeast  by  serpentines,  chloritic  schists,  micaceous 
schists,  and  argillites,  which  are  typical  Huronian  rocks.  The  inter- 
mediate position  of  the  Huronian  seems  to  show  that  it  is  below  the 
Montalban.  The  Primal  and  Auroral  are  the  Lower  Taconic  of  Em- 
mons. South  of  the  Susquehanna,  South  Mountain  rocks  again  ap- 
pear and  stretch  southward  to  the  Potomac.  They  here  consist  of 
Montalban  and  Huronian  rocks.  In  the  southern  part  of  Pennsyl- 
vania are  bedded  petrosilex  rocks,  often  jasper-like,  which  are  asso- 
ciated with  characteristic  rocks  of  the  Huronian  series,  to  which  they 
are  all  referred. 
Prime,8  in  1878,  describes  gneiss  and  mica  schist  in  Lehigh  County 
as  Laurentian.  A  little  west  of  Seller's  quarry  the  Potsdam  sand- 
stone and  Laurentian  rocks  are  seen  in  contact.  The  dips  of  the  two 
seem  to  be  conformable,  but  this  may  be  wrong,  as  the  exposure  is 
