260  TRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  '  [bull. 86. 
sediments  of  different  characters  are  seen  to  cut  across  the  prominent 
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  fold- 
ing. 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  quan- 
tities of  later  eruptives.  Granite  is  found  in  the  slate  area  as  mapped 
by  Newton. 
A  study  of  the  boundary  between  the  slate  and  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  the  contact  and  dynamic  meta. 
morphism  due  to  the  intrusion  of  the  granitic  rock.  The  effect  hag 
extended  for  several  miles  from  the  main  granite  area.  The  normal 
foliation  of  the  slates  and  schists  is  north  and  south,  and  this  was  pro- 
duced by  foMing  earlier  than  the  intrusion  of  the  granite.  About  the 
granite  area  both  sedimentation  and  this  earlier  foliation  were  de 
stroyed  and  a  more  prominent  newer  foliation  produced.  Also,  in  the 
northern  part  of  the  slates  is  a  considerable  area  about  Dead  wood 
which  .is  now  as  crystalline  as  the  schist  area  of  the  south.  This  b 
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  beer 
wholly  destroyed  for  considerable  areas  by  a  newer  and  more  promi 
nent  schistose  structure.  The  slates  and  schists  can  not  then  be  di 
yided  into  two  series  with  the  surface  distribution  and  upon  the  litho 
logical  differences  given  by  Newton. 
The  mica-slates,   mica-schists,   and  mica-gneisses  are  found  to  b< 
clastic  rocks,  the  processes  of  change  from  their  original  clastic  com 
dition  to  their  present  crystalline  one  being  traced  out.    Associate 
with  the  clastic  rocks  are  other  green  crystalline  schists  which  an 
metamorphosed  basic  eruptives  which  were  probably  intruded  beforj 
the  earlier  folding  of  the  slates.     Corroborating  Newton's  conclusion' 
it  is  said  that  the  Black  hills  rocks  exhibit  a  remarkable  lithologitJ 
analogy  to  certain  of  the  iron-bearing  series  of  the  lake  Superior  regioj  J.] 
which  in  the  past  have  been  included  under  the  term  Huronian.     Whi 
this  correlation  is  not  beyond  doubt,  there  is  no  question  that  tlies 
series  in  common  belong  to  the  Algonkian. 
SUMMARY    (>K    KKSl'LTS. 
The  summary  of  Newton's  work  is  altogether  unsatisfactory  becau 
of  the  very  large  amount  of  material  contained  in  this  report.    1j 
