410  t>RE-CAMBRlAN   ROCKS    OF    NORTH   AMERICA.  [bull.  86. 
division  is  made  between  the  Huronian  and  the  next  following  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  Hnronian  area  of 
the  crystalline  schists. 
Rand,163  in  1889,  describes  a  section  of  the  crystalline  rocks  from  the 
Triassic  of  Chester  county,  Pennsylvania,  to  the  Cretaceous  of  New 
Jersey,  passing  through  Philadelphia.  The  rocks  are  doubtfully  re- 
ferred to  various  horizons,  running  from  the  Laurentian  to  the  Hudson 
river. 
LITERATURE   OF   MARYLAND. 
Ducatel  and  Alexander,164  in  1834,  describe  the  Primary  rocks  as 
one  of  the  chief  divisions.  These  include  the  following  formations : 
Granite,  gneiss,  limestone,  and  serpentine. 
Aikin,165  in  1834,  states  that  granite  and  primitive  schists  are  inter- 
mingled in  every  possible  manner  in  the  region  west  of  Baltimore, 
the  dips  being  with  a  good  deal  of  regularity  toward  the  southeast. 
Succeeding  the  primitive  rocks  are  transition  slates,  sandstones, 
limestones,  and  greywackes  interstratified  with  transition  limestones. 
Duoatel,166  in  1839,  states  that  the  limestones  of  Harford  and  Balti- 
more counties  occur  in  the  valleys.  In  the  northwest  part  of  these 
counties  the  rocks  are  argillites,  which  pass  into  talcose  slates,  and 
these  are  succeeded  by  granitic  aggregates  in  which  hornblende  is  the 
prevailing  rock. 
Tyson,167  in  1860,  classifies  the  rocks  of  Maryland  into  those  of 
igneous  and  aqueous  origin.  In  the  former  are  granite,  syenite,  mas- 
sive quartzite,  porphyry,  amygdaloid,  trap  (including  hornblende  rock 
or  amphibolite),  and  serpentine.  The  rocks  of  aqueous  origin  include 
chemical  deposits,  among  which  are  limestone  and  dolomite;  mechanical 
deposits,  among  which  are  sandstone,  conglomerate,  breccia,  clay-slate, 
shale,  and  clay;  and  metamorphic  rocks,  among  which  are  gneiss,  mica- 
slate,  hornblende- slate,  talc-slate,  quartzite,  granular  limestone,  and 
dolomite.  The  rocks  of  igneous  origin  are  defined  as  those  which  give 
no  evidence  of  stratification.  These  are  found  in  the  area  about  Balti- 
more, mingled  with  the  sedimentary  rocks.  In  the  limestones  in  many 
cases  the  stratification  has  been  obliterated.  Gneiss  is  the  most  largely 
developed  of  the  rocks  in  the  central  part  of  the  state.  While  there  is 
usually  ample  evidence  of  stratification  in  gneiss,  in  some  localities  it 
has  been  so  much  altered  by  the  joint  action  of  heat  and  intrusive 
forces  as  to  have  nearly  obliterated  its  stratification  planes  and  cause  it 
to  resemble  granite.  The  four  lowest  formations  of  Maryland  are 
eruptive;  the  fifth  formation  is  composed  of  gneiss,  mica-slate,  and 
hornblende- slate,  which  includes  the  intrusive  rocks  of  the  first  four 
formations  and  a  portion  of  the  limestone.  These  rocks  occur  as  a 
belt  in  Cecil,  Harford,  Baltimore,  Howard,  and  Montgomery  counties, 
