THE   CORDILLERAS.  767 
still  covered  with  the  crystalline  schists,  afterward  as  foothills,  and 
at  last  very  abundant,  disputing  with  the  eruptive  rocks  the  pre- 
ponderance in  the  structure  of  the  Sierra  Madre,  or,  to  put  it  better, 
the  complicated  aggregate  of  sierras  which  covers  all  of  the  southern 
part  of  the  Republic. 
A  completely  distinct  phase  is  exhibited  by  the  extensive  group  of 
c^stalline  schists,  considered  provisionally  by  us  as  Archean,"  which 
appear  principally  in  the  southern  part  of  the  State  of  Puebla,  in 
the  districts  of  Chietla  and  Acatlan.  The  formation  of  Acatlan, 
which  can  be  well  studied  on  the  banks  of  the  river  of  that  name, 
consists  at  the  base  of  augen  gneiss  of  a  dark  color,  upon  which  rests 
a  mass  of  green  schists,  very  diversified  and  much  exfoliated  and  sub- 
jected to  very  frequent  foldings  and  dislocations. 
Even  though  there  were  no  connection  between  these  crystalline 
slates  of  Acatlan  and  the  Archean  slates  of  the  coast  belt,  it  would 
still  appear  to  us  that  the  Acatlan  formation  can  belong  to  a  division 
more  recent  or  to  a  higher  member  of  the  Archean.  As  we  have  al- 
ready said,  this  opinion  should  be  later  corroborated. 
Upon  the  crystalline  rocks  of  Acatlan  rest  patches  of  sedimentary 
Triassic  beds  and  the  Cretaceous  limestone,  and  they  in  turn  are  hid- 
den by  recent  eruptive  rocks. 
In  closing  this  summary  enumeration  of  the  classes  of  rocks  that 
make  up  our  Archean  formation,  we  may  cite  the  observation  which 
Dr.  E.  Bose  has  been  kind  enough  to  communicate  to  us,  taken  from 
his  work  on  the  geology  of  the  Isthmus  of  Tehuantepec,6  which  is 
now  being  printed. 
Bose  divides  the  isthmus  into  three  regions — the  plain  of  the  gulf 
coast  toward  Coatzacoalcos,  the  sierra  of  the  center,  and  the  plain 
of  the  Pacific  coast  toward  Tehuantepec.  The  crystalline  schists  arc 
found  exclusively  in  the  last  region  and  extend  as  far  as  the  slopes 
of  the  central  sierra. 
The  slates  constitute  a  true  complex,  in  that  Bose  was  not  able  to 
discover  the  proper  relation  of  superposition  nor  the  stratigraphic 
features,  as  much  on  account  of  the  effects  of  erosion  a-  from  the  fact 
that  they  appear  exposed  only  in  isolated  sections  because  of  the 
recent  sediments  which  cover  them.  The  slates  lie  oriented  in  all 
directions  and  with  diverse  inclinations.  The  crystalline  complex  of 
Tehuantepec  appears  to  present  types  of  slate  similar  to  those  of  the 
region  of  Ejutla  and  Miahuatlan,  chiefly  biotite  gneiss,  amphibole 
gneiss,  mica  slate,  green  chloritic  schist,  muscovite  gneiss,  etc. 
In  contact  with  the  schists  are  gneissic  granites,  and  bet  ween  the 
masses  which  are  exposed  in  the  plain  of  the  coast  are  found  perthitic 
granite,  perhaps  ;i-  ancient  as  that  of  Acapulco,  biotite  granite,  dior- 
aP6rfil  geol6gico  de  Acapulco  6   Veracruz. 
6Res<'fia  acerca  de  la  geplogia  de  Chiapas  y  Tabasco:    Bol.   [nst.  geol.  de  M6rico  No.  20. 
