420  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA,  [bull.'jB6. 
structure  of  a  sediment  or  a  laminated  rock,  and  which  often  appears 
like  the  dark-green  slates  of  the  Taconic  system.  This  singular  stinc- 
ture  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,  in  these  cases,  are  the  farthest  removed  from  rocks  of  this 
description,  and  with  which  water  has  had  nothing  to  do.  The  lamina 
are  sometimes  as  thin  as  paper,  and  from  their  appearance  can  not  be 
distinguished  from  the  slates  referred  to.  These  dikes  are  bounded  by 
walls  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  difficult 
to  determine  the  line  of  demarkation  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 
and  mica-slate  and  hornblende-rocks  with  laminae  parallel  with  them, 
but  still  they  have  many  characters  which  belong  only  to  the  eruptive 
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  nowr  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.  It  might  be  doubted  whether  they  were  sediments  at 
all  were  it  not  that  they  are  associated  with  conformable  pebbly  beds, 
which  is  the  only  x>roof  that  these  rocks  are  really  sedimentary. 
The  Taconic  rocks  are  divided  into  lower  and  upper  parts.  The  lower 
series  contains  talcose  slates  with  white  and  brown  sandstones  and 
quartz,  with  granular  limestones  and  associated  slates,  and  with  these 
occur  hornblende,  which  makes  it  difficult  to  determine  where  the  pri- 
mary rocks  end  and  the  Taconic  begins,  especially  when  the  pebbly 
beds  are  absent.  Vitrified  quartz  can  not  be  regarded  as  always  an  ig- 
neous product,  but  rather  as  a  deposit  of  silica  from  chemical  solution. 
The  materials  composing  the  belts  of  detritus  are  apparently  derived 
from  the  granites,  as  shown  by  the  fact  that  the  quartz  and  feldspar  of; 
these  rocks  are  distinguishable  in  the  brecciated  conglomerates. 
Kerr,  J9t  in  1867,  finds  that  the  slates  of  western  North  Carolina 
have  an  average  strike  of  N.  50  °  E.,  the  dips  being  high  to  the  south- 
east, for  the  most  part  about  05°.  The  greatest  variations  in  strike 
and  dip  are  in  the  central  area,  where  the  strata  are  contorted  and 
folded  to  an  unusual  degree.  This  region  extends  from  the  Black 
mountains  to  the  southeastern  corner  ot  Clay  county.  This  central 
area  is  the  axis  of  the  state  and  is  composed  for  the  most  part  of  granitic 
