PIEDMONT  PLATEAU  AND  PORTIONS  OF  THE  APPALACHIANS.      675 
being  seen  to  rest  on  the  slope  of  the  ridge,  with  northwest  undulating 
dips  on  the  edges  of  the  southeastward-dipping  older  rocks.  In  other 
cases  the  Primal  beds,  thrown  into  southeast  dips  in  the  hills  which 
flank  the  Blue  Ridge,  are  made  to  underlie,  with  more  or  less  approxi- 
mation to  conformity,  the  older  rocks  forming  the  central  part  of  the 
mountain.  But  even  in  those  instances  it  is  not  difficult  to  discern  the 
true  relations  of  the  strata.  Examples  of  the  phenomena  are  the 
sections  exposed  at  Vestals,  Gregorys,  Snickers,  and  Manassas  gaps, 
and  Jeremies  Run,  in  the  northern  part  of  the  Blue  Ridge;  and  at 
Dry  Run,  Turks,  Tye  River,  White-,  James  River,  Point  Lookout, 
Fox  Creek,  and  White-top  Mountain  gaps,  in  the  middle  and  south- 
western prolongation  of  the  chain. 
.  Campbell  (J.  L.  and  II.  D.),55  in  1884,  conclude  from  an  examina- 
tion of  the  Snowdon  quarries  that  the  core  of  the  Blue  Ridge  is  an 
igneous  mass  belonging  to  the  Archean,  and  that  upon  its  north- 
western slope  are  unconformable  beds  of  slates,  sandstones,  and  con- 
glomerates which  are  Potsdam  or  Cambrian.  They  are  in  a  highly 
metamorphosed  condition,  and  Avere  regarded  by  Rogers  as  Huronian, 
and  by  the  authors  as  pre-Cambrian,  but  the  discovery  of  fossils  in 
them  has  definitely  determined  their  age.  The  slaty  cleavage  of  the 
quarries  sometimes  corresponds  with  the  planes  of  original  bedding 
or  stratification,  but  more  frequently  is  more  or  less  oblique  to  the 
strata. 
Geiger  and  Keith,50  in  1801,  in  discussing  the  structure  of  the  dis- 
trict about  Harpers  Ferry,  state  that  between  the  Cambro-Silurian 
shale  and  the  granite  schist  there  is  an  unconformity  of  the  ordinary 
type  of  deposition. 
Darton,57  in  1892,  finds  Ordovician  fossils  in  the  crystalline  slates 
and  schists  of  the  Piedmont  Plain  of  Virginia,  these  rocks  having 
been  previously  regarded  as  Huronian. 
Darton,35  in  1894,  maps  and  describes  the  geology  of  the  Fred- 
ericksburg quadrangle,  in  Virginia  and  Maryland.  See  summary  in 
section  2,  Maryland,  etc.,  pages  667-668. 
AVatson,58  in  1902,  describes  the  copper-bearing  rocks  of  the  Virgi- 
lina  copper  district  of  Virginia  and  North  Carolina  and  sliows  the 
adjacent  rocks  to  be  pre-Cambrian  metamorphosed  andesite  as- 
sociated with  corresponding  volcanic  elastics.  All  are  collectively 
referred  to  as  greenstones,  and  are  thought  to  be  similar  to  green- 
stones described  as  occurring  along  the  Atlantic  coast  region  from 
eastern  Canada  to  Georgia  and  from  Alabama  to  the  Lake  Superior 
region. 
See  summaries  of  work  of  YValcott,  Keith,  and  others  under  Mary- 
land, section  2  of  this  chapter. 
