GENERAL   ACCOUNT    OF   THE    PRE-CAMBRIAN    ROCKS.  55 
Throughout  northern  China  there  are  extensive  areas  of  basement 
complex,  which  is  tentatively  assigned  to  the  Archean  on  the  ground 
of  its  complex  internal  structure,  probably  predominant  igneous  char- 
acter, and  position  at  the  base  of  the  pre-Cambrian  terranes.  It  in- 
cludes limited  amounts  of  marble  and  schists  which  are  of  sedimen- 
tary origin,  but  is  dominantly  composed  of  gneisses,  believed  to  have 
been  derived  largely  from  igneous  rocks,  and  is  intruded  by  various 
igneous  rocks,  among  which  granite  predominates.  The  principal 
occurrences  of  this  complex  which  have  been  studied  are  in  Shan- 
tung, northwestern  Chi-li,  and  northern  Shan-si. 
The  later  pre-Cambrian  formations  of  China  have  been  discrimi- 
nated best  in  the  Wu-t'ai-shan  Range  in  northern  Shan-si.  This 
range  presents  at  least  three  and  possibly  four  pre-Cambrian  groups 
later  than  the  Archean,  divided  by  unconformities.  Of  the  three 
which  may  be  definitely  recognized  the  older  two  constitute  a  sequence 
of  quartzite,  biotite  schist,  muscovite  schist,  and  chlorite  schist,  with 
more  or  less  marble.  The  structures  are  complex,  embracing  schis- 
tosity,  isoclinal  folding,  and  overthrust.  Two  groups  are  recognized 
as  clearly  distinct — the  Nan-t1ai  group  of  conglomerate,  quartzite, 
and  limestone,  and  the  Si-t'ai  group,  consisting  prevailingly  of  chlo- 
rite schist.  The  latter  is  believed  to  be  the  younger.  A  third  group, 
designated  the  Shi-tsui  group,  consists  largely  of  biotite  and  musco- 
vite schists  and  quartzite.  It  is  probably  older  than  either  of  the 
others,  but  may  represent  some  part  of  the  Nan-t'ai.  The  collective 
name  retained  for  the  Nan-t'ai,  Si-t'ai,  and  Shi-tsui  groups  is  Wu-t'ai 
series,  originally  given  by  Von  Richthofen. 
Unconformably  overlying  the  extensively  altered  Wu-t'ai  series 
is  a  series  of  slates  and  limestones,  which  are  but  slightly  metamor- 
phosed and  only  moderately  folded.  The  unconformity  is  recognized 
chiefly  by  the  difference  of  metamorphism  and  of  structure,  as  im- 
mediate contacts  have  not  been  oberved.  The  younger  sequence  has 
received  the  name  Hu-t'o  series.  The  exact  succession  has  not  been 
ascertained,  but  the  base  of  the  series  appears  to  include  conglom- 
erate, quartzite,  and  gray  slates;  and  that  part  of  the  series  which  is 
believed  to  constitute  the  upper  half  consists  chiefly  of  flinty  gray 
limestones  with  interbedded  slates.  The  total  thickness  amounts  to 
2,000  feet  or  more. 
Throughout  the  northern  part  of  the  province  of  Chi-li  there  oceans 
a  pre-Cambrian  limestone,  which  has  received  the  name  Ta-yang  or 
Xan-k'ou  limestone.  At  the  observed  contacts  it  rests  unconformably 
upon  the  Archean  with  but  a  thin  layer  of  argillite  between  them, 
and  it  is  unconformably  overlain  by  strata  of  lower  or  middle  Cam- 
brian age.  This  limestone  may  be  the  representative  of  the  Hu-t'o 
series,  but  as  the  occurrences  are   widely  separated   and    the  strati- 
