nhtse.]  EASTERN   UNITED    STATES.  411 
id  are  bounded  on  the  northwest — or,  more  correctly  speaking,  pass 
Y  insensible  shades  of  difference  into  the  talcose  slates.  Near  the 
mthwestern  limit  the  prevailing  rock  is  gneiss,  which  is  interlami- 
ited  with  hornblende-slate.  In  proceeding  northwest  mica-slate  in- 
cases in  quantity,  and  in  passing  still  farther  this  mica-slate  passes 
to  talc-slate.  The  metainorphic  limestones  are  found  in  two  ranges  • 
ie  first,  the  gneisses  and  mica-slates ;  and  second,  the  talcose  slates. 
Williams,168  in  1886,  describes  the  gabbros  and  associated  horn- 
endic  rocks  of  Maryland.  These  are  all  found  to  be  of  igneous  origin 
id  the  schistose  hornblendic  rocks  the  result  of  metamorphism. 
Williams,169  in  1891,  describes  the  structure  of  the  Piedmont  plateau 
Maryland.  The  western  part  is  a  semicrystalline  area  consisting 
'  phyllites,  sandstones,  marbles,  and  but  few  eruptive  rocks.  The 
^stern  area  is  completely  crystalline.  The  sedimentary  rocks  in- 
ude  biotite-gneiss,  biotite-muscovite- gneiss,  muscovite-gneiss,  mica- 
hist,  quartz-schist,  conglomeratic  quartz-schist  and  dolomitic  marble. 
rithin  this  area  are  very  numerous  eruptive  rocks,  including  granites, 
leisses,  gabbros,  diorites,  and  basic  rocks,  such  as  pyroxenite,  lherzo- 
;e,  etc.  Two  sections  are  described  in  detail.  In  the  semicrystalline 
cks  a  cleavage  is  developed  which  much  obscures  the  bedding,  and 
e  succession  may  be  repeated  many  times  by  folds  and  faults.  Be- 
reen  the  semicrystalline  and  completely  crystalline  rocks  there  is  a 
mewhat  abrupt  passage.  The  structure  of  the  western  area  can  be 
counted  for  by  a  single  period  of  folding,  while  the  eastern  area,  as 
Lown  by  its  implicated  structure,  must  have  been  wrenched,  folded, 
Ld  faulted  at  different  times.  It  is  concluded  that  the  eastern  area  is 
mposed  of  rocks  far  more  ancient  than  the  western,  which  extend 
ider  the  latter,  forming  the  floor  upon  which  they  were  deposited, 
lis  hypothesis  accounts  for  the  difference  in  crystalline  character  be- 
reen  the  rocks  of  the  two  areas,  for  the  abruptness  of  their  contact, 
Ld,  since  both  series  have  been  subjected  to  a  folding  together,  for 
eir  apparent  conformity  along  their  contact.  As  to  the  age  of  the 
cks,  it  is  probable  that  the  Paleozoic  should  include  all  the  semicrys- 
lline  schists,  while  the  noncrystalline  rocks  east  of  them  would  be 
signed  to  the  Algonkian  or  Archean. 
Keyes,169  in  1891,  gives  as  a  supplement  to  the  preceding  a  section 
ross  the  Piedmont  plateau  of  Maryland.  In  the  Frederick  limestone 
the  western  semicrystalline  rocks  are  fossils  of  several  types  charac- 
ristic  of  the  Trenton,  and  the  entire  series  of  limestones  and  shales 
obably  represent  the  Chazy,  Trenton,  and  Hudson  river  formations. 
ist  of  the  western  semicrystalline  rocks  are  contorted  gneisses,  with 
neral  westerly  dips,  which  are  cut  by  basic  and  acid  rocks  and 
lich  are  believed  to  have  been  originally  granitic,  but  through  the 
ency  of  enormous  orographic  pressure  have  been  squeezed  into  their 
Bsent  gneissic  condition,  as  shown  by  the  mechanical  deformations 
ough  which  the  grains  have  gone. 
