730  PRE-CAMBRTAN    GEOLOGY    OF    NORTH   AMERICA. 
eastern  slate  area,  the  pebbles  of  which  have  suffered  extensive  de- 
formation by  compression.  The  granitic  rocks  of  the  schist  area  do 
not  penetrate  the  slates  of  the  newer  series,  although  this  is  not  devoid 
of  eruptive  rocks.  The  newer  series  of  slates  is  correlated  with  the 
Taconian  of  western  New  England.  The  conclusion  is  reached  that 
the  granite,  instead  of  being  of  eruptive  origin,  is  pegmatitic. 
Carpenter,44  in  1888,  states  that  the  unconformity  supposed  to 
exist  by  Newton  between  the  eastern  slate  and  the  western  schist 
series  is  supported  by  an  observation  upon  Spring  Creek  east  of  Hill 
City.  A  huge  dike  of  igneous  rock,  a  thousand  feet  broad  in  places, 
is  described  as  passing  through  the  entire  length  of  the  eastern  series. 
Van  Hise,45  in  1890,  finds  that  the  prominent  structures  of  the 
Black  Hills,  which  have  heretofore  been  taken  as  bedding,  are  second- 
ary structures.  As  evidence  of  this  is  the  fact  that  alternating  bands 
of  sediments  of  different  characters  are  seen  to  cut  across  the  promi- 
nent lamination.  Sometimes  these  belts  are  conglomeratic  and  the 
pebbles  are  deformed  by  pressure.  The  longer  axes  of  the  pebbles 
are  parallel  to  the  slaty  or  schistose  structure,  but  the  belts  as  a 
whole  cut  across  this  structure.  The  fact,  cited  by  Newton,  that 
there  are  persistent  belts  of  quartzite  parallel  with  each  other,  also 
indicates  duplication  by  folding.  The  thickness  of  the  Archean  is, 
then,  unknown,  instead  of  being  more  than  100,000,  as  supposed 
by  Newton.  The  crystalline  schists  are  in  a  broad  zone  about  the 
granite  area,  striking  parallel  to  it  and  dipping  away  from  it,  and  in 
the  northern  hills  there  are  great  quantities  of  later  eruptives.  Gran- 
ite is  found  in  the  slate  area  as  mapped  by  Newton. 
A  study  of  the  boundary  between  the  slate  and  the  schist  series 
leads  to  the  conclusion  that  there  is  a  gradation  from  the  slates  to  the 
schists  rather  than  an  abrupt  change.  The  foliation  of  the  schists 
about  the  granite  is  secondary  and  is  caused  by  th&  contact  and 
dynamic  metamorphism  due  to  the  intrusion  of  the  granitic  rock.  The 
effect  has  extended  for  several  miles  from  the  main  granite  area.  The 
normal  foliation  of  the  slates  and  schists  is  north-south,  and  this  was 
produced  by  folding  earlier  than  the  intrusion  of  the  granite.  About 
the  granite  area  both  sedimentation  and  this  earlier  foliation  were  de- 
stroyed and  a  more  prominent  foliation  was  produced.  Also,  in  the 
northern  part  of  the  slates,  about  Deadwood,  is  a  considerable  area 
which  is  now  as  crystalline  as  the  schist  area  of  the  south.  This  is 
taken  to  be  due  to  the  abundant  later  intrusives  here  found.  We  thus 
have  in  this  region  evidence  of  an  original  bedding  which  is  nearly 
obliterated  by  a  prominent  slaty  cleavage,  and  both  of  these  have 
been  wholly  destroyed  for  considerable  areas  by  a  newer  and  more 
prominent  schistose  structure.  The  slates  and  schists  can  not,  then, 
be  divided  into  two  series  with  the  surface  distribution  and  upon  the 
lithological  differences  given  by  Newton. 
