PIEDMONT  PLATEAU  AND  PORTIONS  OF  THE  APPALACHIANS.      665 
chemical  deposits,  among  which  are  limestone  and  dolomite;  mechan- 
ical 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  Baltimore,  mingled  with  the  sedimentary  rocks.  In 
the  limestones  in  many  places  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  intru- 
sive 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,  and  are  bounded  on  the  northwest  by — or,  more 
correctly  speaking,  pass  by  insensible  shades  of  difference  into — the 
talcose  slates.  Near  the  southwestern  limit  the  prevailing  rock  is 
gneiss,  which  is  interlaminated  with  hornblende  slate.  Toward  the 
northwest  mica  slate  increases  in  quantity,  and  finally  passes  into  talc 
slate.  The  metamorphic  limestones  are  found  in  tAvo  belts — (1)  the 
gneisses  and  mica  slates,  and  (2)  the  talcose  slates. 
Williams,20  in  1886,  describes  the  gabbros  and  associated  horn- 
blendic  rocks  of  Maryland.  These  are  all  found  to  be  of  igneous  ori- 
gin and  the  schistose  hornblendic  rocks  the  result  of  metamorphism. 
Williams,27  in  1891,  describes  the  structure  of  the  Piedmont  Pla- 
teau in  Maryland.  The  western  part  is  a  semicrystalline  area  consist- 
ing of  phyllites,  sandstones,  marbles,  and  but  few  eruptive  rocks. 
The  eastern  area  is  completely  crystalline.  The  sedimentary  rocks 
include  biotite  gneiss,  biotite-muscovite  gneiss,  muscovite  gneiss,  mica 
schist,  quartz  schist,  conglomeratic  quartz  schist,  and  dolomitic  mar- 
ble. Within  this  area  are  very  numerous  eruptive  rocks,  including 
granites,  gneisses,  gabbros,  diorites,  and  basic  rocks,  such  as  pyrox- 
enite,  lherzolite,  etc.  Two  sections  are  described  in  detail.  In  the 
semicrystalline  rocks  a  cleavage  is  developed  which  much  obscures  the 
bedding,  and  the  succession  may  be  repeated  many  times  by  folds  and 
faults.  Between  the  senlicrystalline  and  completely  crystalline  rocks 
there  is  a  somewhat  abrupt  passage.  The  structure  of  the  western 
area  can  be  accounted  for  by  a  single  period  of  folding,  while  the  east 
ern  area,  as  shown  by  its  implicated  structure,  must  have  been 
wrenched,  folded,  and  faulted  at  different  times.  It  is  concluded  thai 
the  eastern  area  is  composed  of  rocks  far  more  ancient  than  the  west 
ern,  which  extend  under  the  latter,  forming  the  floor  upon  which  they 
