672  PRE-CAMBRIAN    GEOLOGY   OF    NORTH   AMERICA. 
The  second  occurrence  of  Baltimore  gneiss  is  found  in  an  anticlinal 
dome  15  miles  long  and  5  miles  broad  lying  on  either  side  of  the 
Northern  Central  Railroad  10  miles  south  of  the  Mason  and  Dixon 
line  and  20  miles  north  of  Baltimore. 
Three  smaller  areas  occur  in  the  vicinity  of  Baltimore;  two  of 
them  are  portions  of  anticlinal  domes  which  are  either  completely 
inclosed  by  overlying  sediments  or  are  cut  off  by  faults  and  igneous 
rocks,  while  the  third,  underlying  the  northwestern  part  of  Balti- 
more city,  is  entirely  surrounded  by  gabbro  and  other  igneous  masses 
and  is  overlain  in  great  measure  by  the  Coastal  Plain  deposits. 
Between  Baltimore  and  Washington,  along  the  boundary  line  be- 
tween Montgomery  and  Prince  George  counties,  occurs  another  area 
of  Baltimore  gneiss  which  up  to  the  present  time  has  not  been  fully 
investigated. 
The  rocks  in  each  of  these  areas  consist  of  highly  crystalline 
gneisses  composed  of  quartz,  feldspar,  and  mica  with  accessory  min- 
erals which  are  so  distributed  as  to  produce  well-marked  gray  banded 
gneisses,  the  individual  bands  of  which  vary  from  a  fraction  of  an 
inch  upward,  the  average  breadth,  however,  being  small.  Some  of 
these  beds  are  highly  quartzose,  resembling  a  micaceous  quartzite; 
others  are  rich  in  biotite  or  hornblende,  producing  dark  schists  which 
in  a  hand  specimen  are  indistinguishable  from  metamorphosed 
igneous  masses.  Within  the  areas  of  Baltimore  gneiss  are  numerous 
small  bodies  of  metamorphosed  granites  and  more  basic  igneous  rocks 
which  have  been  intruded  into  the  gneiss  and  subsequently  meta- 
morphosed until  they  are  practically  indistinguishable  from  it.  The 
diiferences  in  character  can  now  and  then  be  recognized,  but  it  has 
not  been  found  possible  to  carry  the  mapping  of  these  small  igneous 
intrusions  from  one  exposure  to  another,  and  many  of  them  are  so 
small  that  they  could  not  be  represented  on  maps  of  the  scale  em- 
ployed. 
The  structural  character  of  the  Piedmont  from  Trenton  southward 
to  southern  Virginia  is  similar  throughout  to  the  Appalachian 
structure  of  the  less  metamorphosed  Paleozoics  to  the  west.  That  is, 
one  may  recognize  within  the  Piedmont  a  series  of  long  and  narrow 
folds  and  arches  trending  parallel  to  the  trend  of  the  Appalachians. 
An  exception  to  this  rule  is  noticeable  in  the  central  Maryland  area, 
which  lies  toward  the  center  of  a  local  curve  in  the  Appalachian 
structure,  where  the  structural  forms  are  more  nearly  circular  than 
they  generally  are  in  the  Appalachians. 
Clark  and  Mathews,44*1  in  1906,  give  a  general  account  of  the 
geology  of  Maryland  that  is  essentially  similar,  in  regard  to  the  pre- 
Cambrian,  to  the  articles  by  Mathews  covered  by  the  two  preceding 
summaries.  The  sedimentary  part  of  the  Baltimore  gneiss  is  doubt- 
fully referred  to  the  Algonkian.  The  igneous  rocks  are  grouped 
under  the  heading  "  Paleozoic- Archean." 
