22  PRE-CAMBRIAN   ROCKS    OF   NORTH    AMERICA.  [bull.  86. 
In  regard  to  regions  like  the  Appalachians  and  California,  in  which 
post-Cambrian  rocks  have  become  completely  crystalline  and  have  been 
for  many  years  and  are  yet  confused  with  pre-Cainbrian  rocks,  sum- 
maries unavoidably  extend  beyond  the  proper  scope  of  this  paper.  Of 
series  which  were  in  the  past  supposed  to  be  pre  Cambrian,  but  which 
have  been  demonstrated  to  be  Cambrian  or  post-Cambrian,  the  fact 
is  mentioned,  but  the  literature  which  concerns  them  is  not  sum- 
marized, since  it  does  not  fall  within  the  scope  of  this  paper.  The  par- 
ticular position  which  such  formations  shall  take  in  Cambrian  or  post- 
Cambrian  time  is  a  subject  for  others  to  consider. 
Of  necessity  the  eruptives  which  have  not  been  differentiated  from 
the  pre-Cambrian  have  to  be  considered.  Oftentimes  it  is  quite  proba- 
ble that  eruptives  noted  belong  to  post-Cambrian  time.  No  paper  it? 
summarized  bearing  upon  the  unmistakable  Cambrian  or  post-Cam- 
brian eruptives  unless  it  has  a  direct  bearing  upon  the  character  or  re- 
lations of  the  associated  rocks  of  pre-Cambrian  age. 
All  references  to  literature  are  given  at  the  ends  of  the  respective 
chapters,  the  reference  notes  having  continuous  numbers.  Tu  the  dis- 
cussions closing  sections  or  chapters  and  in  the  general  chapter  cita- 
tions are  not  repeated.  The  original  source  of  any  statement  attrib- 
uted to  an  author  may  always  be  found  by  the  aid  of  the  index,  where 
the  name  of  each  author  is  followed  by  references  to  the  pages  where 
his  work  is  summarized. 
The  terms  group,  system,  series,  are  used  with  the  strati  graphical 
significance  given  them  by  the  International  Geological  Congress.  The 
corresponding  chronological  terms  era  and  period  are  used.  Forma- 
tion is  used  as  one  of  the  members  of  a  series,  as  quartzite  formation 
or  limestone  formation  of  the  Huronian  series.  The  term  u  crystalline 
schist "  is  rigidly  confined  to  rocks  which  have  a  "  completely  crystal- 
line interlocked  texture,  which  is  possessed  of  a  schistose  parting  due 
to  a  parallel  or  foliated  arrangement  of  the  mineral  ingredients,  or  of 
aggregations  of  these  ingredients."  The  finely  banded  gneisses  are 
typical  examples.  All  rock  masses  which  within  themselves  show  in- 
dubitable evidence  of  clastic  origin  are  excluded  from  the  crystal- 
line schists  and  are  regarded  as  semi  or  partially  crystalline.  A  clastic 
or  semi-crystalline  formation  may  grade  injto  a  crystalline  schist. 
-  In  the  summaries  of  results  and  in  the  general  discussion  (Chap- 
ter VIII),  unless  otherwise  stated,  the  term  Cambrian  is  delimited  below 
by  the  Olenellus  fauna.  The  term  Algonkian  is  a  system  term,  covering 
all  recognizable  pre-Cambrian  clastic  rocks.  The  term  Archean  is  a  co- 
ordinate-system term,  covering  all  pre- Algonkian  rocks.  It  therefore 
includes  only  completely  crystalline  rocks,  but  does  not  include  all  rocks 
of  this  kind,  as  noncrystalline  rocks  of  eruptive  and  sedimentary  ori- 
gin may  occur  in  Algonkian  or  post- Algonkian  time.  The  propriety  of 
these  usages  will  appear  in  what  follows. 
