PIEDMONT  PLATEAU  AND  PORTIONS  OF  THE  APPALACHIANS.       667 
Darton,31  in  1892,  finds  Ordovician  fossils  in  the  crystalline  slates 
and  schists  of  the  Piedmont  Plain  of  Virginia,  these  rocks  having 
been  previously  regarded  as  Huronian. 
Williams  and  Clark,32  in  1893,  describe  and  map  the  geology  and 
physical  features  of  Maryland.  The  pre-Cambrian  rocks,  described 
by  Williams,  form  the  eastern  or  noncrystalline  division  of  the  Pied- 
mont Plateau  region  of  Maryland,  crossing  the  State  in  a  general 
southwest  direction  from  the  southeast  coiner  of  Pennsylvania  and 
the  north  end  of  Delaware.  These  rocks  are  but  a  part  of  the  great 
crystalline  plateau  which  extends  from  New  York  to  Alabama  along 
the  eastern  base  of  the  Appalachians.  Toward  the  east  the  pre- 
Cambrian  rocks  of  Maryland  plunge  under  Coastal  Plain  deposits 
and  toward  the  west  they  form  the  floor  to  support  the  Paleozoic 
strata  of  the  Appalachians,  reappearing  in  the  granitic  and  volcanic 
rocks  of  South  Mountain  of  Pennsylvania.  The  noncrystalline  rocks 
are  divisible  into  six  types,  three  of  which — gabbro,  peridotite  or 
pyroxenite,  and  granite — are  of  undoubted  eruptive  origin,  and  three 
of  which — gneiss,  marble,  and  quartz  schist — while  showing  no  cer- 
tain evidence  of  clastic  structure,  are  believed  to  be  sedimentary. 
The  prevailing  rock  is  gneiss,  closely  associated  with  marbles  and 
quartz  schists,  forming  an  intricate  complex.  The  complex  shows 
evidence  of  great  dynamic  action,  the  rocks  having  been  almost  com- 
pletely recrystallized.  The  eruptive  rocks  are  all  younger  than  the 
gneisses.  The  gabbro  is  the  oldest,  followed  by  the  peridotite  or 
pyroxenite,  and  the  youngest  is  the  granite.  The  granites  are  as  a 
rule  medium-grained  biotite  granites,  but  they  frequently  take  the 
form  of  pegmatite. 
Keyes,33  in  1893,  holds  that  the  granites  of  Maryland  are  eruptive, 
since  these  rocks  indiscriminately  cut  across  the  other  igneous  rocks 
of  the  region,  as  well  as  the  gneiss;  because  they  hold  inclusions  of 
the  other  rocks  of  the  region ;  because  the  rocks  cut  show  contact  phe- 
nomena, and  because  a  microscopical  examination  shows  that  they 
possess  all  the  characters  of  rocks  cooled  from  fusion. 
Grimsley,34  in  1894,  describes  and  maps  the  rocks  of  a  part  of 
Cecil  County  in  northwestern  Maryland.  The  rocks  are  granite,  dio- 
rite,  and  staurolitic  mica  schist.  The  staurolitic  mica  schist  is  regarded 
as  a  sedimentary  rock.  In  this  the  granite  gneiss  is  intrusive,  as 
shown  by  the  fact  that  branching  dikes  and  apophyses  penetrate  the 
adjoining  schists  and  slates,  producing  pronounced  contact  effects 
upon  them.    The  diorite  occurs  in  dikes  in  the  granite. 
Darton,35  in  1894,  maps  and  describes  the  geology  of  the  Fred- 
ericksburg quadrangle,  in  Virginia   and    Maryland.     In  the   north 
western  and  western  parts  are  granite,  gneiss,  and  schist,  ami  ;i  belt 
of  rock  called  the  Quantico  slate,  which  locally  appears  to  grade  into 
siliceous  mica  schist  or  gneiss.     It  is  about  three-quarters  of  a  mil" 
