766  PRE-CAMBRIAN    GEOLOGY    OF    NOETH   AMERICA. 
In  all  this  region  of  Guerrero  and  Oaxaca,  where  the  Archean  for- 
mation acquires  the  greatest  extension,  the  granitic  rocks  are  always 
prominent,  appearing  in  all  places  as  the  skeleton  or  the  resistant 
frame,  which  the  effects  of  the  erosion  on  the  schists  have  caused  to  be 
towering.  Such  masses,  rising  in  the  midst  of  the  fields  of  Archean 
schist,  differ  in  age  and  in  lithological  character. 
There  are  granitic  masses  so  ancient  that  they  can  hardly  be  sepa- 
rated from  the  Archean  rocks,  and  it  might  even  be  thought  that 
these  form  the  base  of  all  the  formations.  Such,  for  example,  are  the 
alkaline  and  perthitic  granites  of  the  Sierra  de  Acapulco,  which 
appear  to  have  been  much  disturbed,  the  same  as  the  schists.  This 
granite  contains  intrusions  or,  more  exactly,  segregations  of  green 
dioritic  rocks  similar  to  those  which  are  seen  in  the  ancient  granites 
of  Ascutney  Mountain,  described  by  Daly.0 
The  perthitic  granite  takes  on  a  schistose  appearance  (gneissic 
granite)  and  afterward  passes  into  a  true  gneiss. 
Very  ancient  granite  can  likewise  be  identified  in  some  regions  of 
Oaxaca,  as  in  the  district  of  Ejutla  and  Puerto  Angel  and  in  the 
Isthmus  of  Tehuantepec. 
More  recent  granite  rocks  crossing  the  Archean  rocks  are  very 
numerous  and  diversified.  We  may  mention  here  as  most  abundant 
biotite  and  pyroxenic  granites,  granite  diorites,  diorites,  and  some 
porphyrites  and  felsites. 
The  dikes  in  the  Archean  rocks  are  very  abundant,  although  not  of 
great  thickness.  In  order  of  age — since  some  are  crossed  by  others — 
we  may  cite  pegmatite  dikes  with  garnet,  with  white  mica,  or  with 
black  tourmaline,  dikes  of  pure  quartz,  and  dikes  of  a  green  aphanitic 
rock  of  a  heterogeneous  composition. 
We  have  already  mentioned  the  fact  that  up  to  the  present  time 
there  has  been  no  identification  in  Mexico  of  the  Paleozoic  rocks  rest- 
ing directly  upon  the  Archean  rocks. 
From  what  we  have  seen  thus  far  it  appears  that  during  the  Paleo- 
zoic era  a  large  part  of  the  Archean  rocks  were  emerged  and  exposed 
to  destructive  erosion.  Only  in  the  south  of  Mexico,  in  the  State  of 
Chiapas,  Sapper 1}  found  Carboniferous  formations  and  probably 
Devonian.  In  almost  all  parts  of  the  slopes  of  the  Sierra  Madre  del 
Sur  the.  Archean  crystalline  rocks  disappeared  under  the  limestone, 
which  is  exposed  in  massive  bands  and  which  more  than  once  has 
been  referred  to  the  Cretaceous,  but  this  determination  of  the  age  of 
the  limestone  must  yet  be  proved  in  many  places. 
These  Mesozoic  rocks  appear  to  rise  toward  the  sierra  from  the 
coast,  first  as  small  patches  crowning  the  mass  of  the  granite,  in  part 
a  Daly,  R.  A.,  Geology  of  Ascutney  Mountain,  Vermont :  Bull.  U.  S.  Geol.  Survey  No. 
209,  1903.     Mechanics  of  igneous  intrusions  :  Am.  Jour.  Sci.,  4th  ser.,  vol.  15,  1903. 
h  Op.  cit.  ;  also  Sobre  la  geografia  fisica  y  la  geologfa  de  la  peninsula  de  Yucatan  :  Bol. 
Inst.  geol.  de  Mexico  No.  3,   1896. 
