PIEDMONT  PLATEAU  AND  PORTIONS  OF  THE  APPALACHIANS.       673 
SECTION  3.     THE  VIRGINIAS. 
SUMMARY  OF  LITERATURE. 
Cornelius,43  in  1818,  finds  west  of  the  Secondary  formations  ranges 
of  granites,  schists,  and  other  Primitive  rocks.  The  Blue  Ridge  is  the 
dividing  line  between  the  granite  and  the  limestone  country  to  the 
west, 
Rogers  (W.  B.),4g  in  1840,  describes  the  southern  district  east  of 
the  Blue  Ridge  as  occupied  mostly  by  rocks  of  very  ancient  date, 
most  of  them  believed  to  be  Primary.  A  part  of  them  are  in  irregu- 
lar masses,  and  others  have  regular  stratification,  but  all  are  alike 
considered  of  metamorphic  origin.  Aside  from  these  there  occur 
igneous  rocks.  The  more  important  metamorphic  rocks  are  granite, 
syenite,  gneiss,  mica  slate,  talc  slate,  argillaceous  slate,  pseudo  gneiss 
or  gneissic  sandstone,  soapstone  rocks,  micaceous  and  talcose  lime- 
stones, and  marbles.  By  pseudo  gneiss  or  gneissoid  sandstone  is  meant 
rocks  which  resemble  the  truly  crystalline  rocks  but  which  plainly 
betray  their  sedimentary  origin  by  the  rounded  character  of  the 
quartz  and  other  constituents.  The  igneous  rocks  cut  the  shales  and 
sandstones  of  the  Middle  Secondary. 
Rogers  (W.  B.),47  in  1841,  gives  the  geological  occurrences  of  the 
Primary  and  metamorphic  rocks.  In  these,  beds  of  limestone  are 
included  at  various  points.  Quartz  slate  and  quartzite  are  found  in 
the  Bull  Run  Mountains  and  other  localities. 
Fontaine,48  in  1875,  describes  several  sections  of  crystalline  rocks 
which  are  regarded  as  pre-Silurian.  Among  them  are  argillite,  green- 
stone, and  syenite.  At  a  tunnel  the  contact  of  the  Silurian  with  the 
argillite  is  beautifully  exposed,  and  the  great  contrast  of  the  two  sys- 
tems is  well  shown. 
Fontaine,49  in  1875,  describes  the  central  part  of  the  Blue  Ridge  as 
consisting  of  coarse  granites  and  gneisses  of  Laurentian  age.  Along 
the  eastern  slope  of  the  syenite  is  a  formation  of  argillites  which  is 
covered  by  a  series  of  mica  slates,  schists,  and  gneisses.  The  axis  is 
occupied  by  talcose  limestones,  quartzites,  mica  slates,  and  hydromica 
slates  which  closely  resemble  those  in  Berkshire  County,  Mass.  In 
this  belt  are  probably  two  systems,  one  older  than  the  Primordial,  and 
the  other  metamorphosed  Silurian.  The  unconformity  which  exists 
between  the  syenite  and  the^argillite  apparently  shows  the  latter  to  be 
Iluronian,  although  its  age  is  not  positively  determined. 
Campbell  (J.  L.),50  in  1879,  states  that  the  Archean  rocks  of  the 
Blue  Ridge  are  granite  and  syenite.  They  underlie  the  stratified 
rocks  of  the  region,  but  are  probably  more  recent  than  they,  being- 
thrown  upward  through  them.  'Hie  bedded  rocks  resting  upon  the 
syenite  are  much  metamorphosed  and  gneissoid  in  character.     These 
55721— Bull.  360—09 43 
