THE    CORDILLERAS.  765 
On  both  sides  of  the  chain  of  mountains  connected  with  those  of 
the  Bay  of  Acapulco  the  crystalline  schists  extend  for  a  great  dis- 
tance, represented  particularly  by  biotite  gneiss,  which  pass  down 
ward  near  the  shore  and  on  the  slopes  of  Sierra  de  Acapulco  to  a 
gneissic  granite,  while  toward  the  interior,  on  the  slopes  of  the 
heights  of  the  Sierra  Madre,  there  are  intercalated  thin  beds  of  mus- 
covite  gneiss.  While  the  crystalline  schists  have  various  folds,  it 
appears  that  the  main  body  of  the  formation  has  an  inclination  to- 
ward the  north,  or  a  little  deviated  to  the  northeast  or  to  the  north- 
west. About  50  kilometers  from  the  beach  toward  the  banks  of  the 
river  Papagallo,  which  crosses  the  highway  of  Acapulco,  this  incli- 
nation of  the  slates  is  seen.  Here  they  are  not  biotite  gneiss,  but  a 
series  of  green  schists,  leafy  sericitic  and  chloritic  rocks,  to  which 
the  gneiss  changes  gradually  until  it  predominates  a  little  farther 
in  the  interior  at  the  mountain  of  Macho,  near  Tierra  Colorada, 
with  a  thick  formation  of  mica  schist,  which  in  this  hill  rests  against 
a  very  ancient  granite. 
OAXACA,    PUEBLA,    AND    CHIAPAS. 
The  biotite  gneiss,  the  schists  which  rest  upon  it,  and  the  mica 
schists  of  the  upper  part  continue  with  analogous  characteristics 
along  the  Archean  belt  to  the  east  toward  the  State  of  Oaxaca.  where 
the  petrographic  character  of  the  formation  greatly  changes,  but  we 
are  not  yet  able  to  judge  of  the  true  importance  of  each  type  of  schist ; 
nor  can  we  determine  the  real  order  of  succession,  because  it  is  xvvy 
difficult  to  follow  the  series  in  its  true  order  on  account  of  the  ad- 
vanced wasting  by  erosion  and  the  complicated  and  numerous  strati- 
graphic  accidents  which  the  great  crystalline  mass  shows. 
Aguilera  found  recently,  on  the  road  from  Oaxaca  to  Puerto  Angel . 
the  crystalline  complex  not  much  diversified,  being  reduced  to  some 
few  types  of  gneiss,  crossed  by  dikes  and  intrusive  rocks  of  the  class 
of  biotite  granite  and  diorite,  while  Felix  and  Lenk."  while  surveying 
the  road  between  Ejutla  and  Miahuatlan,  also  in  the  State  of  Oaxaca, 
saw  (in  order  of  succession,  commencing  with  the  most  ancient) 
muscovite  gneiss  (which  is  of  little  importance  in  Guerrero),  graph- 
itic gneiss,  and  pyroxene  gneiss.  In  some  other  places  of  Oaxaca 
these  authors  find  different  successions  of  the  crystalline  complex,  or 
local  exposures  of  pyroxene,  graphitic  amphibole,  and  biotite  gneiss. 
(Jirault  h  found  graphitic  gneiss  and  biotite  gneiss  with  intercala- 
tions of  lenses  and  layers  of  beautiful  crystalline  marble,  with  ampin- 
bole  mica  and  pyroxene  (cipolina),  between  Etla  and  San  Miguel  de 
las  Peras  and  in  regions  not  far  from  those  examined  by  Felix  and 
Lenk. 
"  Beitrage  zur  Geologie  rind  Palaeontologie  cl<-r  Republik  Mexiko,  pt.  "J.  L899,  pp.  L58    159 
bInforme  de  la  zona  minera  de  San  Miguel  de  la*  Peras;  L'.ol.  Agr.  y  Mln.,  October,  L892, 
