PIEDMONT  PLATEAU  AND  PORTIONS  OF  THE  APPALACHIANS.       669 
parallel  banding,  water  segregations.     The  majority,  however,   ar< 
igneous,  as  is  shown  by  all  of  the  phenomena  of  intrusive  rocks. 
Clark,40  in  1897,  describes  the  geology  and  physical  features  of 
Maryland.  This  account  is  essentially  the  same  as  that  published  by 
Williams  and  Clark  in  1893,  already  summarized  (p.  667).  Here, 
however,  the  crystalline  rocks  are  classed  as  Archean  and  Algonkian, 
both  of  which  are  included  under  the  general  term  Archean.  The 
statement  is  made  that  there  is  no  positive  evidence  that  rocks  of  the 
earliest  portion  of  Archean  time  (meaning  Archean  proper)  are  rep- 
resented in  Maryland,  although  a  part  of  the  gneiss  complex  may 
represent  it.  The  Algonkian  period,  however,  is  represented  by  many 
varieties  of  rock.  The  rapidity  with  which  the  crystalline  rocks  fur- 
nished sediments  for  the  overlying  formations  points  to  their  high 
elevation  in  Archean  time. 
In  the  western  division  of  the  Piedmont  Plateau  region  of  Mary- 
land, Algonkian  rocks  are  infolded  with  the  Paleozoic  deposits  of 
Montgomery,  Frederick,  and  Carroll  counties.  They  consist  of  a 
single  type  resembling  the  metamorphosed  basic  volcanic  rocks 
(Catoctin  schist)  of  the  Blue  Ridge  district. 
Clark,41  in  1897,  describes  the  physical  features  and  geology  of 
Maryland,  and  gives  a  sketch  of  the  development  of  knowledge  con- 
cerning them.  The  description  of  pre- Cambrian  geology  is  essen- 
tially the  same  as  that  given  by  Clark  in  the  preliminary  publication 
of  this  part  of  the  volume  (noticed  above),  and  this  in  turn  is  but 
slightly  different  from  the  account  given  by  Williams  and  Clark  in 
1893  (noticed  on  p.  667).  However,  a  few  minor  changes  may  be 
noted.  The  crystalline  rocks  of  the  Piedmont  Plateau  region,  instead 
of  being  divided  into  six  types  as  before,  are  divided  into  seven  types, 
diorite  being  added  to  the  list.  Rocks  of  the  Archean  period  are 
placed  in  the  table  of  formations  as  doubtfully  present. 
Keith,42  in  1901,  describes  and  maps  the  geology  of  the  Piedmont 
Plateau  of  the  Washington  district.  Igneous  rocks  of  Archean 
age  are  mapped  under  the  following  divisions:  Biotite  granite,  soap- 
stone  and  serpentine  (altering  from  peridotite  and  pyroxenite),  gab- 
bro,  metagabbro,  diorite  and  metadiorite,  granite  (including  gneis- 
soid  granite  and  schistose  granite),  and  Carolina  gneiss  (including 
mica  gneiss,  mica  schist,  and  small  bodies  of  granite,  schistose  granite, 
and  diorite).  In  age  these  rocks  rank  in  the  order  named,  the  Caro- 
lina gneiss  being  the  oldest.  Also  the  relative  areas  of  the  group- 
nearly  correspond  with  their  ages. 
Mathews,43  in  1904,  discusses  the  structure  of  the  Piedmont 
Plateau.  The  Baltimore  gneiss  (Williams's  biotite  gneiss  in  part)  i- 
correlated  with  the  Fordham  gneiss  of  New  York,  the  Baltimore 
gneiss  of  Philadelphia,  and  the   Carolina  gneiss    (in   part)    of  the 
