418  PRE-CAMBRIAN    ROCKS    OF   NORTH    AMERICA.  [bull. 86. 
more  or  less  approximation  to  conformity,  the  older  rocks  forming  the 
central  part  of  the  mountain.  Bnt  even  in  those  instances  it  is  not  dif- 
ficult to  discern  the  true  relations  of  the  strata.  As  examples  of  the 
phenomena  are  the  sections  exposed  at  Yestals,  Gregorys,  Snickers, 
and  Manassas  gaps,  and  Jeremies  run,  in  the  northern  part  of  the  Blue 
ridge;  and  at  Dry  run,  Turks,  Tye  river,  Whites,  James  river,  point 
Lookout,  Fox  creek,  and  White-top  mountain  gaps,  in  the  middle  and 
southwestern  prolongation  of  the  chain. 
Campbell  (J.  L.  and  H.  D.),184  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  northwestern  slope 
are  unconformable  beds  of  slates,  sandstones,  and  conglomerates  which 
are  Potsdam  or  Cambrian.  They  are  in  a  highly  metamorphosed  condi- 
tion, and  were  regarded  by  Eogers  as  Huronian,  and  by  the  authors  as 
pre-Cambrian,  but  the  discovery  of  fossils  in  them  has  definitely  deter- 
mined their  age.  The  slaty  cleavage  of  the  quarries  sometimes  corre- 
sponds with  the  planes  of  original  bedding  or  stratification,  but  more 
frequently  is  more  or  less  oblique  to  the  strata. 
Geigker  and  Keith,185  in  1891,  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. 
LITERATURE   OF   NORTH   CAROLINA. 
Olmsted,186  in  1824,  describes  as  parallel  with  the  freestone  and 
coal  formations  a  great  slate  formation  which  extends  across  the  state 
from  northeast  to  southwest,  being  about  20  miles  wide,  running 
through  Person,  Orange,  Chatham,  Eandolph,  Montgomery,  Cabarrus, 
Anson,  and  Mecklenburg  counties.  Within  this  district  are  found 
numerous  beds  of  porphyry,  soapstone,  serpentine,  greenstone,  and 
whetstone.  From  Halifax  to  Person  courthouse  hardly  any  kind  of 
rock  but  granite  is  met. 
Olmsted,187  in  1825,  more  fully  describes  the  great  slate  formation, 
which  includes  argillite,  greenstone,  porphyry,  uovaeiilite,  petrosiJexj 
hornstone,  black  steatite,  ^syenite,  etc.  Between  the  great  slate  forma- 
tion and  the  Blue  ridge  is  a  granitic  district,  various  limestone  beds, 
and  a  transition  formation.  The  granitic  district  occupies  the  whole 
country,  with  subordinate  exceptions,  from  the  slate  formation  to  the 
Blue  ridge.  The  term  granitic  as  here  used  embraces  gneiss  and  mica- 
slate  as  well  as  granite.  Among  the  subordinate  beds  non  earc  so 
numerous  as  greenstone.  In  Stokes  and  Surrey,  in  connection  with 
the  iron  ores,  are  numerous  isolated  beds  of  limestone  which  lie  in  niica- 
slate  rocks. 
Mitchell,188  in  1820,  states  that  of  the  primitive  rocl*  of  North  Car- 
olina, the  more  ancient  lie  farther  west  and  the  more  recent  in  the  mid- 
land counties.     Those  of  the  eastern  division  are  highly  crystalline  in 
