670  PRE-CAMBRIAN   GEOLOGY   OF   NORTH   AMERICA. 
Washington  area,  all  of  Avhich  are  referred  to  the  pre-Cambrian. 
He  concludes  that : 
1.  The  older  rocks  of  the  Piedmont  consist  of  both  sedimentary 
and  igneous  types  which  since  their  formation  have  been  more  or  less 
met  a  morphosed . 
2.  The  metamorphosed  sediments  include  banded  micaceous  and 
hornblende  gneisses  of  pre-Cambrian  age ;  a  more  or  less  intermittent, 
thin-bedded,  generally  tourmaline-bearing  quartzite  of  Cambrian 
age;  and  intermittent  dolomitic  marble  or  magnesian  limestone  of 
Cambro-Ordovician  age ;  and  a  series  of  mica  schists  and  the  gneisses 
of  Ordovician  age.  Above  these  occur  a  somewhat  intermittent, 
poorly  developed,  quartzitic  conglomerate  and  the  Peach  Bottom 
slates. 
3.  The  igneous  rocks  consist  of  an  immense  gabbro  mass,  intruded 
by  numerous  large  bodies  of  granite  and  metarhyolite,  and  accom- 
panied by  numerous  more  basic  serpentinized  bodies.  These  various 
masses  represent  stages  in  a  single  extended  period  of  igneous  activity. 
4.  The  time  when  this  activity  took  place  was  later  than  early 
Silurian  and  earlier  than  late  Carboniferous,  probably  in  the  early 
part  of  this  interval. 
5.  The  chief  structural  features  of  the  region  are  the  metamorph- 
ism  and  contant  schistosity  and  the  broader  folding  of  the  different 
rocks. 
6.  The  metamorphism  of  the  rocks,  especially  of  the  banded 
gneisses,  probably  commenced  prior  to  the  intrusion  of  the  gabbro 
and  granite,  and  was  accentuated  by  them  in  the  eastern  portion  of 
the  plateau. 
7.  The  folding  of  the  region  is  of  the  Appalachian  type,  the  rocks 
occurring  in  several  long,  more  or  less  parallel  folds,  with  few  faults 
and  but  occasional  overturned  folds. 
8.  The  eastern  and  western  areas  are  probably  of  the  same  age, 
differences  in  metamorphism  being  due  to  the  large  bodies  of  deep- 
seated  intrusives  on  the  east  and  the  smaller  bodies  of  surface  vol- 
canics  on  the  west. 
9.  The  sequence  found  in  Maryland  may  be  recognized  from  Wash- 
ington to  Trenton  and  in  the  region  north  of  New  York. 
Mathews,44  in  1905,  concludes  that  the  four  formations  recognized 
by  Bascom  in  the  Philadelphia  area  may  be  traced  across  Maryland 
and  that  they  probably  constitute  the  bulk  of  the  rocks  forming  the 
Piedmont  of  Virginia  which  have  heretofore  been  mapped  as  a  unit. 
Within  the  Maryland  Piedmont,  so  far  as  studied  in  detail,  there  are 
no  means  of  locating  accurately  the  position  in  the  stratigraphic 
column  of  the  four  formations — Baltimore  gneiss,  Setters  quartzite, 
Cockeysville  marble,  and  Wissahickon  schists — but  on  either  end  of  a 
continuous  extension  of  these  formations  Ave  have  a  sequence  into 
