IMKDMONT  PLATEAU  AND  PORTIONS  OF  THE  APPALACHIANS.       695 
guished.  According  to  Keith  the  pre-Cambrian  rocks  of  this  area 
are  of  Algonkian  age,  and  comprise  Catoctin  schist  (altered  diabase) 
and  granite.    No  pre-Cambrian  sediments  are  recognized. 
The  Washington  folio  covers  the  adjacent  parts  of  Alexandria 
and  Fairfax  counties,  Va.,  extending  northwest,  west,  and  southwest 
from  the  city  of  Washington.  The  crystalline  rocks  comprised  within 
the  Virginia  portion  of  this  district  are,  according  to  Keith,  of  pre- 
Cambrian  age  and  are  all  referred  to  the  Archean.  The  principal 
types  distinguished  comprise  mica  gneiss,  mica  schist,  granite  gneiss 
diorite  and  metadiorite,  gabbro  and  metagabbro,  soapstone  and  ser- 
pentine, and  biotite  granite. 
In  1D01  Watson  traced  and  described  the  continuation  of  a  belt 
of  ancient  volcanic  rocks  in  North  Carolina  northward  into  Halifax 
County,  Va.,  and  ascribed  to  the  belt  a  pre-Cambrian  age,  probably 
Algonkian.  The  rocks  comprised  altered  andesites  and  the  associated 
tuffs,  and  are  probably  contemporaneous  in  age  with  the  South  Moun- 
tain volcanics  of  Maryland  and  Pennsylvania. 
Excepting  the  several  areas  of  Juratrias  rocks  which  cover  parts 
of  the  Virginia  Piedmont  region,  areas  of  undoubted  metainorphic 
crystalline  sediments  occur  over  parts  of  the  region  not  yet  mapped, 
of  post-Algonkian  age.  A  considerable  part  of  the  Piedmont  com- 
plex, however,  is  composed  of  metainorphic  crystalline  rocks  closely 
similar  to  those  recently  mapped  as  pre-Cambrian,  comprising  origi- 
nal igneous  masses  and  sediments.  Granites,  granite  gneisses,  soap- 
stone,  and  serpentine  make  up  the  principal  types. 
North  Carolina. — No  attempt  is  here  made  at  a  discussion  of  the 
earlier  work  on  the  geology  of  the  crystalline  rocks  of  North  Caro- 
lina by  Emmons  and  Kerr;  only  the  more  recent  work  since  L890  is 
reviewed. 
The  easternmost  belt  of  metamorphic  crystalline  rocks  in  the  State 
passes  eastward  beneath  the  Coastal  Plain  sediments  and  the  narrow 
trough  of  Triassic  sandstones,  and  was  named  by  Kerr  the  Ilnronian 
belt  of  central  North  Carolina.  It  includes  a  complex  of  altered 
sedimentary  and  igneous  masses,  comprising  (1)  argillaceous,  seri- 
eitic,  and  chloritic  metamorphosed  slates  and  schists;  (2)  devit  ri- 
fled ancient  volcanics,  rhyolite,  quartz  porphyry,  metaandesite,  etc., 
and  pyroclastic  breccias  and  tuffs;  (3)  igneous  plutonic  rocks,  gran- 
ite, granite  gneisses,  diorite,  etc. ;  (1)  siliceous  magnesian  Limestones; 
and   (5)  sedimentary  pre- Juratrias  slates. 
The  chloritic  schists  are  in  part  the  equivalents  of  profoundly 
altered  basic  erupt ives.  They  are  less  abundant  than  the  argilla- 
ceous schists,  and  are  often  garnet  i  ferous  and  epidote  bearing.  The 
general  strike  of  the  schistosity  is  N.  20°  to  55°  E.,  and  the  dip 
55°  to  85°  NW. 
