756  PRE-CAMBRIAN    GEOLOGY    OF    NORTH   AMERICA. 
tion  in  the  occurrence  of  these  pre-Cambrian  rocks,  which  here  almost 
disappear,  or  are  found  in  very  limited  areas.  Instead  there  is  in 
this  wide  zone  an  abundance  of  volcanic  rocks,  principally  of  a  recent 
epoch,  the  flows  of  which  at  many  points  have  covered  the  area  to 
the  shores  of  the  ocean. 
In  the  southern  part  of  Mexico,  approximately  in  the  isthmian 
region,  the  pre-Cambrian  rocks  acquire  greater  relative  importance. 
Beginning  as  a  narrow  belt  in  the  vicinity  of  Zihuatanejo,  the  forma- 
tion broadens  gradually  toward  the  southeast  of  Acapulco,  and  con- 
tinues yet  broader  into  the  southern  districts  of  the  State  of  Puebla 
and  the  coastal  region  of  Oaxaca.  Narrowing  again,  the  formation 
trends  toward  the  isthmus  of  Tehuantepec,  and  continues  very  narrow 
in  the  State  of  Chiapas,  whence  a  narrow  belt  curving  toward  the 
Atlantic  crosses  the  territory  of  Guatemala,  as  has  been  demonstrated 
by  the  studies  of  Sapper.a 
APPARENT   DISCONTINUITY   OF   AN   ARCHEAN   BELT. 
Even  in  the  central  part  of  southern  Mexico  and  in  the  isthmian 
region  these  rocks  are  by  no  means  continuous,  since  belts  or  zones 
of  rocks  probably  Mesozoic,  and  also  more  recent  coastal  and  marine 
formations,  cover  them  in  many  places.  Geologically,  we  can  assume 
that  the  Archean  formations  are  continuous  there  and  form  a  general 
basement  in  the  larger  part  of  the  coast  districts  of  the  States  of 
Guerrero,  Oaxaca,  and  Chiapas. 
MEXICAN    PRE-CAMBRIAN    ROCKS    MUST    BE    ARCHEAN. 
We  shall  not  again  use  the  term  pre-Cambrian,  because  it  has  not 
yet  been  possible  to  distinguish  the  different  divisions  of  the  Archean 
in  these  Mexican  formations,  or  even  to  decide  what  part  should 
correspond  to  the  Algonkian,  or  to  recognize  a  lapse  of  time  between 
the  Archean  and  the  Paleozoic.  As  will  be  seen  from  the  map,  we 
have  not  definitely  marked  the  outlines  of  the  areas,  and  have  left 
out  of  consideration  the  patches  of  more  recent  rocks  which  are  to 
be  found  throughout  the  said  areas. 
TECTONIC  IMPORTANCE  OF  THE  ARCHEAN   BELT. 
Some  foreign  geologists  have  been  surprised  at  the  limited  area 
of  the  Archean  rocks  in  Mexico  and  have  given  to  the  known  occur- 
rences of  this  formation  in  Mexico  an  areal  importance  greater  than 
they  possess. 
Felix  and  Lenk  &  have  without  reason  designated  the  southern  belt 
of  the  Archean  rocks  as  the  "  Pacific  Archean  Cordillera ;  "  they  have 
a  Sapper,  Carl,  Grundziige  a.  pliysikalischen  Geographie  von  Guatemala,  Gotha,  1894. 
6  Felix,   J.,   and   Lenk,   H.,   Ueber  die  tektonischen   Verhaltnisse  der  Republik  Mexiko  : 
Zeitschr.  Deutsch.  geol.  Gesell.,  vol.   13,  1892. 
