THE   CORDILLERAS.  759 
ing  the  great  depressions  which  tectonic  accidents  left  in  the  Mesozoic 
rocks,  so  that  the  modern  sediments  hide  these  rocks  in  the  Mesa  Cen- 
tral and  only  the  most  elevated  parts  surmount  them. 
To  us  it  appears  possible  also,  as  we  have  said,  that  in  the  place' 
where  the  crystalline  belt  has  by  its  curvature  resisted  the  continuation 
of  certain  movements  of  a  continental  character  it  has  developed 
a  region  of  maximum  fracture,  where  there  have  arisen  numerous 
volcanoes  and  where  enormous  masses  of  volcanic  lava  have  flowed 
out. 
A  proof  that  the  crystalline  belt  is  not  entirely  rigid  can  be  deduced 
from  the  persistence  of  certain  vibratory  movements  which  take  place 
in  the  Archean  region  of  the  south  of  Mexico  and  which  are  trans- 
mitted to  the  central  volcanic  portion  of  the  country.  On  the  coasts 
of  Guerrero  and  of  Oaxaca,  on  the  Isthmus  of  Tehuantepec,  and  in  the 
submarine  regions  adjacent  are  developed  numerous  earthquakes.  It 
is  a  proved  fact  that  the  Pacific  coasts  of  Mexico  have  undergone  in 
recent  epochs  sensible  relative  movement,  and  a  submersion,  real  or 
apparent,  can  be  verified  now  not  only  in  the  portions  of  the  coast 
covered  by  the  modern  rocks  but  also  in  the  Archean  regions  which 
touch  the  Pacific  coast. 
The  Archean  rocks  are  too  remote  from  the  densely  populated  cen- 
ters of  the  country  to  have  been  studied  and  delimited  with  any  pre- 
cision. Without  doubt  it  can  be  said  that  the  series  of  crystalline 
schists  and  of  granitic  rocks  which  are  found  in  each  section  are  gen- 
erally so  diversified  that  it  is  not  possible  to  correlate  them  to  the 
extent  that  the  Mexican  Archean  areas  can  be  represented  in  the  same 
way  as  those  of  other  parts  of  America  have  been  correlated  by  Van 
Hise.a 
This  inequality  in  the  character  and  succession  of  the  rocks  of  the 
different  Archean  exposures  can  not  be  taken  always  as  a  proof  of 
their  independence,  because  they  have  been  deformed  in  a  very  differ- 
ent manner  and  have  been  subjected  to  a  long-continued  denudation: 
for  during  a  long  period  of  the  Paleozoic  era  this  Archean  belt,  parts 
of  which  we  now  see  exposed,  has  been  exposed  to  erosion,  as  can  be 
judged  from  the  absence  of  sediments  belonging  to  this  era,  which 
would  have  rested  upon  them  if  they  had  been  submerged  in  the 
water.  So  it  can  be  understood  that  the  Archean  rocks  have  suffered 
enormous  erosion,  which  is  also  indicated  by  the  high  intrusive  knobs 
that  jut  out  above  the  area  of  Low  relief  of  the  crystalline  schists. 
PETROGRAPHIC   CHARACTER   OF    ARCHEAN    AREAS. 
Generally  in  the  Archean  areas  of  Mexico  only  schists  appear,  and 
these  are  yn'y  crystalline;  but  in  small  areas  there  are  found  argil- 
"  Van  Bise,  C.  R.,  Correlation  papers.  Archean  and  Algonkian :   Bull,   r.  s    Geo)    Survey 
No.  8G,  1892. 
