694  PRE-CAMBRTAN   GEOLOGY   OF   NORTH   AMERICA. 
rocks  comprising  the  Piedmont  region  in  Virginia  include,  according 
to  Rogers,  granite,  gneiss,  syenite;  micaceous,  chloritic,  talcose,  and 
argillaceous  schists;  talcose,  quartz,  and  argillaceous  slates;  soap- 
stone  rocks;  micaceous  and  talcose  limestones  and  marbles;  pseudo- 
gneiss  or  gneissic  sandstone,  and  quartzite.  These  were  all  considered 
by  Rogers  to  be  of  very  ancient  date,  mostly  Primary  comprising 
both  sedimentary  and  igneous  masses,  and  all  highly  metamorphosed. 
Between  the  years  1874  and  1885  the  principal  work  done  in  the 
region  of  the  Virginia  crystalline  rocks  was  by  Fontaine  and  J.  L. 
and  H.  D.  Campbell,  and  was  confined  principally  to  parts  of  the 
Blue  Ridge  region.  As  indicated  by  these  geologists,  the  principal 
rock  types  are  essentially  the  same  as  those  identified  by  Rogers. 
They  comprise  granite,  syenite,  mica  schists,  micaceous  and  horn- 
blendic  gneiss,  granulite,  mica  slates,  hydromica  slates,  argillite, 
greenstone,  talcose  limestone,  quartzites,  and  conglomerates.  In 
places  unconformities  were  noted,  and  the  rocks  were  classified  as 
pre- Silurian  and  assigned  partly  to  the  Laurentian,  partly  to  the 
Huronian,  and  partly  to  the  Primordial  (Cambrian). 
J.  L.  and  H.  D.  Campbell  described  a  series  of  sandstones  and 
slates  flanking  the  southeast  slope  of  the  Blue  Ridge  in  Amherst 
County,  the  sandstones  of  which  contained  fossil  borings  of  Sco- 
lithus  linearis,  which  definitely  marks  them  as  Cambrian  in  age. 
No  fossils  were  reported  found  in  the  slates  of  this  series,  but  the 
Scolithus  sandstone  was  observed  to  dip  under  the  slates,  which  in- 
dicates that  the  latter  can  not  be  older  than  Cambrian.  These  beds, 
sandstone  and  slate,  had  previously  been  mapped  as  Archean. 
Lower  Silurian  fossils  have  been  reported  by  Darton  and  Walcott 
from  the  slates  composing  the  Arvonia  area  on  James  River  in  the 
northeast  corner  of  Buckingham  County  and  east  of  the  Amherst 
County  area  mentioned  above.  These  slates  were  likewise  referred 
to  the  Archean  prior  to  the  discovery  of  fossils  in  them  by  Darton. 
In  the  extreme  northwest  corner  of  the  Fredericksburg  quadrangle 
Darton  has  mapped  and  described  a  narrow  belt  of  slates,  designated 
the  Quantico  slates,  which  resemble  the  slates  of  the  Arvonia  area  in 
Buckingham  County,  but,  as  Darton  remarks,  the  resemblance  may 
be  fortuitous,  as  fossils  were  not  found  in  them,  and  they  may  or  may 
not  be  of  the  same  age  as  those  of  the  Buckingham  area.  Granites 
and  gneisses,  regarded  as  largely  pre- Cambrian,  occur  on  either  side 
of  the  narrow  belt  of  the  Quantico  slates. 
In  1894  and  1901  the  Harpers  Ferry  and  Washington  folios  were 
published  by  the  United  States  Geological  Survey,  each  covering 
parts  of  the  crystalline  rocks  in  northern  Virginia.  The  Virginia 
section  of  the  Harpers  Ferry  quadrangle  includes  a  part  of  the  Blue 
Ridge  and  a  part  of  the  adjacent  western  margin  of  the  Piedmont 
Plateau.     Rocks  of  both  sedimentary  and  igneous  origin  are  distin- 
