PIEDMONT  PLATEAU  AND  PORTIONS  OF  THE  APPALACHIANS.       659 
iferous  schists,  these  relations  being  well  exposed  at  Chester  Creek. 
The  cleavage  dip  varies  from  75°  to  90°,  but  the  true  dips  are  nearly 
horizontal  and  undulating,  which  fact  tends  to  reduce  the  hypo- 
thetical thickness  of  the  crystalline  rocks  of  southeastern  Pennsyl- 
vania to  a  minimum.  The  serpentines  occupy  shallow  synclinal  basins 
and  are  the  most  recent  of  the  metamorphosed  rocks.  East  of  Schuyl- 
kill River,  outside  of  Delaware  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  schis- 
tose series  one  kind  of  rock  gradually  fades  into  the  next  succeeding 
kind,  which  renders  a  delineation  almost  impossible. 
Frazer,16  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. 
Frazer,17  in  1886,  describes  the  Archean  rocks  of  York  County. 
The  lowest  members  of  the  Archean  series  here  found  are  the  Huro- 
nian  schists,  which  have  a  thickness  of  14,400  feet.  A  somewhat 
arbitrary  division  is  made  between  the  Huronian  and  the  next  follow- 
ing age,  the  rocks  of  which  are  denominated  Azoic  schists  or  phyllites, 
as  they  can  not  be  certainly  assigned  either  to  the  Archean  or  to  the 
Paleozoic.  A  belt  of  them  is  found  on  either  side  of  the  broad 
Pluronian  area  of  the  crystalline  schists. 
Rand,18  in  1889,  describes  a  section  of  the  crystalline  rocks  from  the 
Triassic  of  Chester  County,  Pa.,  to  the  Cretaceous  of  New  Jersey, 
passing  through  Philadelphia.  The  rocks  are  doubtfully  referred  to 
various  horizons,  running  from  the  Laurentian  to  the  Hudson  River. 
Lesley,19  in  189k2,  gives  a  summary  sketch  of  the  pre-Cambrian 
rocks  of  Pennsylvania,  the  facts  being  taken  from  the  detailed  State 
reports.  The  Highland  belt  of  New  Jersey  and  Pennsylvania;  the 
Reading  and  Durham  hills;  areas  in  Chester.  Bucks,  Montgomery, 
and  Delaware  counties;  and  an  area  on  Schuylkill  River  are  placed 
in  the  Archean.  All  are  regarded  as  sedimentary  in  origin,  because 
of  the  presence  of  marble,  apatite,  and  iron  ore.  The  newer  gneiss 
of  the  Philadelphia  belt,  the  Azoic  formations  of  York,  Chester,  and 
Lancaster  counties,  and  the  South  Mountain  rocks  are  not  definitely 
referred  to  any  system.  The  term  Huronian  must  be  used  simply  as 
a  proper  and  private  name  for  a  series  of  rocks  exposed  along  a  part. 
of  the.  northern  boundary  of  the  United  States.  Should  a  similar 
series  appear  in  some  other  region  and  he  called  Huronian  on  account 
of  the  resemblance,  the  name  would  have  no  value  whatever,  unless 
we  should  imagine  that  in  a  so-called  Huronian  age  the  whole  surface 
of  the  planet  was  stuccoed  with  a  certain  formation  and  received  suc- 
cessive coats  of  other  kinds  of  rock  in  after  aires.     The  most  dissim- 
