PIEDMONT  PLATEAU  AND  PORTIONS  OF  THE  APPALACHIANS.       079 
sumed  the  structure  of  a  sediment  or  a  laminated  rock,  often  appear- 
ing like  the  dark-green  slates  of  the  Taconic  system.  This  singular 
structure  of  an  eruptive  rock  is  interesting  and  important,  as  it 
proves  that  it  may  be  produced  in  rocks  which  have  been  regarded 
as  sediments  but  which  are  the  farthest  removed  from  rocks  of  this 
description  and  with  which  water  has  had  nothing  to  do.  The  laminae 
are  sometimes  as  thin  as  paper,  and  from  their  appearance  can  not  be 
distinguished  from  the  slates  referred  to.  These  dikes  are  bounded 
by  Avails  of  granite,  and  are  frequently  only  from  6  to  10  inches  wide. 
The  mineral  veins  are  generally  found  on  the  borders  of  the  granite 
areas,  usually  within  1  or  2  miles  of  the  slate.  This  western  belt  is 
10  to  14  miles  wide. 
Among  laminated  pyrocrystalline  rocks  are  placed  gneiss,  mica 
schist,  talcose  slates,  hornblende,  and  certain  limestones.  It  is  diffi- 
cult to  determine  the  line  of  demarcation  between  gneiss  and  granite, 
as  frequently  there  are  passage  beds  connecting  one  with  the  other. 
As  to  the  pyrocrystalline  limestones,  they  certainly  occur  among  the 
gneiss,  mica  slate,  and  hornblende  rocks,  with  laminae  parallel  with 
them,  yet  they  have  many  characters  which  belong  only  to  the  erup- 
tive rocks. 
Resting  upon  the  laminated  pyrocrystallines,  with  the  granite  as  a 
substratum,  are  rocks  of  sedimentary  origin  which  are  supposed  to  be 
Azoic.  Above  these  are  other  rocks  which  have  been  in  the  past  re- 
garded as  Azoic,  but  are  now  found  to  be  fossiliferous.  The  older 
deep-seated  sediments  are  sometimes  distinguished  with  difficulty 
from  the  true  Primary  series,  their  lithological  characters  very  often 
belonging  to  the  same  order.  Indeed,  the  only  proof  that  these  rocks 
are  sedimentary  is  their  association  with  conformable  pebbly  beds. 
The  Taconic  rocks  are  divided  into  lower  and  upper  parts.  The 
lower  series  contains  talcose  slates  with  white  and  brown  sandstones 
and  quartz,  and  granular  limestones  and  associated  slates,  and  with 
these  occur  hornblende,  which  makes  it  difficult  to  determine  where 
the  Primary  rocks  end  and  the  Taconic  begins,  especially  when  the 
pebbly  beds  are  absent.  Vitrified  quartz  can  not  be  regarded  as  al- 
ways an  igneous  product,  but  rather  as  a  deposit  of  silica  from  chem- 
ical solution.  The  materials  composing  the  belts  of  detritus  are  ap- 
parently derived  from  the  granites,  as  shown  by  the  fact  that  tin1 
quartz  and  feldspar  of  these  rocks  are  distinguishable  in  the  brec- 
ciated  conglomerates. 
Kerr,67  in  L8673  finds  that  the  slates  of  western  North  Carolina 
have  an  average  strike  of  X.  50°  E.,  the  dips  being  high  to  the  south- 
east, for  the  most  part  about  65°.  The  greatest  variations  in  strike 
and  dip  are  in  the  central  area,  where  the  strata  are  contorted  mid 
folded  to  an  unusual  degree.  This  region  extends  Prom  the  Black 
Mountains  to  the  southeast  corner  of  Clay  Count  v.     This  cent  rai  area 
