van  hise.]  EASTERN    UNITED    STATES.  409 
and  gneisses  at  Soutli  mountain.  The  ridges,  instead  of  being  simple 
anticlines,  are  a  series  of  anticlines  and  synclines.  At  Morgan  hill 
there  is  discordance  between  the  dips  of  the  Potsdam  and  the  gneiss, 
showing  apparent  nonconformity.  The  syenite  rocks  underlie  the  lime- 
stones, which  may  represent  residual  material  which  has  not  been  re- 
moved by  erosion.  The  crystalline  character  of  these  outlying  ridges 
of  limestone  may  be  explained  by  the  fact  that  the  material  has  been 
buried  30,000  or  40,000  feet  below  the  surface.  At  Chestnut  hill  gap  on 
the  Delaware,  the  Potsdam  sandstone  is  sometimes  vitreous  and  over  it 
are  limestones  changed  into  crystalline  dolomites  holding  serpentines. 
In  contact  with  a  dike  of  coarse  granite  near  the  south  side  of  the 
gap  the  slates  are  changed  into  chlorite,  mica-slate,  and  hornblende- 
slates,  but  in  the  coarser  grits  the  original  pebbles  are  seen. 
Hall,  (Charles  E.)  158  in  1883,  describes  many  localities  of  slates, 
gneisses,  and  granites  in  the  South  mountain  area. 
D'Invtlliers,159  in  1883,  states  that  the  existence  of  anticlinal  and 
synclinal  folds  in  the  South  mountain  belt  of  Berks  county  is  suggested 
by  the  alternate  anticlinal  and  synclinal  belts  of  limestone  and  slate, 
but  it  is  not  conclusively  proved,  for  these  formations  belong  to  differ- 
ent systems  of  rocks,  and  no  doubt  lie  unconformably  upon  the  older 
mountain  rocks.  The  South  mountain  rocks  are  gneisses  and  granites, 
which  are  of  two  kinds,  a  distinctly  stratified,  thick  bedded,  massive 
gneiss,  and  a  stratified  syenite  where  hornblende  is  predominant.  The 
eroded  edges  of  the  Potsdam  sandstone  run  along  the  northern  slope 
of  the  belt  overlying  the  gneissoid  rocks. 
Hall  (C.  E.),160  in  1885,  places  the  syenites  of  Delaware  county  with 
the  Laurentian.  Overlying  these  are  the  micaceous  and  garnetiferous 
schists,  these  relations  being  well  exposed  at  Chester  creek.  The  cleav- 
age dip  varies  from  75°  to  90°,  but  the  true  dips  are  nearly  horizontal 
and  undulating,  which  fact  tends  to  reduce  the  hypothetical  thickness  of 
the  crystalline  rocks  of  southeastern  Pennsylvania  to  a  minimum.  The 
serpentines  occupy  shallow  synclinal  basins  and  are  the  most  recent  of 
the  metamorphosed  rocks.  East  of  the  Schuylkill  river,  outside  of  Del- 
aware county,  the  schists  rest  upon  the  upturned  edges  of  the  Potsdam 
and  limestones,  proving  the  relative  age  conclusively.  The  serpentines, 
mica-schists,  and  gneisses  are  regarded  as  more  recent  than  the  Hudson 
river  group.  In  this  schistose  series  one  kind  of  rock  gradually  fades 
into  the  next  succeeding  kind,  which  renders  a  delineation  almost  im- 
possible. 
ErAzer,161  in  1885,  states  that  at  Hendersons  station,  in  the  Phila- 
delphia region,  there  is  an  unconformable  contact  of  the  limestone  with 
the  sandstone,*  and  that  in  the  section  here  there  is  a  series  of  gentle 
folds  rather  than  a  monoclinal  structure,  as  made  out  by  Hall. 
Erazer,162  in  1886,  describes  the  Archean  rocks  of  York  county.  The 
lowest  members  of  the  Archean  series  here  found  are  the  Huronian 
schists,  which  have  a  thickness  of  14,400  feet.     A  somewhat  arbitrary 
