404  PRE-CAMBRLAN    ROCKS    OF    NORTH    AMERICA.  [bull.  86. 
the  limestone  is  white,  but  changes  steadily  with  rapid  gradations  into 
the  blue  limestone.  Sandstones  and  quartzites  of  identical  character 
underlie  both  the  white  and  blue  limestone  and  bind  them  together. 
The  one  distinguishing  fact  which  separates  the  white  from  the  blue  lime- 
stone is  the  presence  of  eruptive  rocks.  It  is  therefore  concluded  that 
the  two  are  identical.  As  the  blue  limestone  belongs  to  the  Cambrian, 
it  is  concluded  that  in  this  region  there  are  no  Archean  limestones,  as 
has  been  supposed. 
LITERATURE   OF  PENNSYLVANIA. 
Finch,146  in  1824,  finds,  near  Easton,  syenite,  serpentine,  and  transi- 
tion limestone,  transition  granite,  transition  clay- slate,  and  transition 
sandstone. 
Finch,147  in  1828,  finds  a  section  from  Chads  fork  to  Westchester  to 
include  gneiss,  mica-slate,  hornblende- slate,  primitive  sandstone,  and 
transition  quartz  rock. 
Eogers  (H.  D.),148  in  1858,  gives  a  systematic  account  of  the  meta- 
morphic  rocks  of  Pennsylvania.  These  are  divided  into  three  main 
divisions:  the  gneissic  series  proper,  or  Hypozoic;  Azoic,  or  those  des- 
titute of  relics  of  life,  and  Paleozoic.  The  Hypozoic  rocks  only  are 
placed  with  the  primary.  The  Azoic  schists  are  regarded  as  newer 
than  the  Hypozoic,  because  of  differences  in  the  position  of  the  two  sets 
of  strata,  in  condition  of  metamorphism,  and  in  manner  of  plication. 
The  former  dip  almost  invariably  to  the  southeast,  while  the  gneiss  in 
many  localities  has  no  symmetrical  folding.  These  dissimilarities  imply 
essential  differences  in  the  directions  and  dates  of  the  crust  movements. 
The  Azoic  rocks,  however,  when  they  show  the  maximum  amount  of  meta- 
morphism, simulate  in  mineral  aspect  and  structure  those  of  the  gneissic 
series.  The  old  strata  are  then  separated  into  three  systems  by  two 
main  horizons,  the  lower,  a  physical  break  between  the  Hypozoic  and 
Azoic  ;  the  upper,  a  life  limit  denoting  the  first  advent,  so  far  as  discov- 
ered, of  organic  beings. 
The  gneissic  rocks  are  separated  structurally  into  three  districts : 
First,  the  area  running  southwestward  from  Trenton,  through  Phila- 
delphia; second,  the  area  between  the  Schuylkill  and  the  Susquehanna, 
north  of  the  first  area;  and,  third,  the  South  mountain  region,  a  contin- 
uation of  the  Highlands  of  New  Jersey.  The  Philadelphia  belt  is  inter- 
sected very  extensively  by  eruptive  rocks,  such  as  granite,  greenstone, 
syenite,  and  trap.  The  second  or  middle  belt  is  sometimes  called  the 
mica-schist  belt,  because  of  the  amount  of  this  mineral  which  it  con- 
tains. The  upper  or  northern  belt  of  gneiss  is  regarded  as  a  part  of 
the  lower  Primal  rocks  and  as  resting  unconformably  upon  the  upper 
gneissic  group,  the  belief  being  based  upon  the  manner  of  the  flexure 
of  the  two  formations  rather  than  upon  actual  unconformable  contacts. 
In  the  Philadelphia  belt  there  is  a  general  prevalence  of  the  north- 
ward dip  of  the  strata,  varying  generally  from  30°  to  50°.    At  Fair- 
