THE   CORDILLERAS.  757 
not  taken  into  account  the  little  part,  if  any,  which  these  rocks  have 
in  the  structure  of  the  mountain  range  that  we  call  the  "  Sierra 
Madre  del  Sur,"  which  extends  between  the  river  Balsas  and  the 
table-lands  of  Puebla  and  Oaxaca  on  the  one  side  and  the  low, 
slightly  undulating  lands  of  the  coasts  of  Guerrero  and  Oaxaca  on  the 
other.  With  much  less  reason  could  we  speak  of  an  Archean  Sierra 
or  Cordillera  in  the  west  of  Mexico  in  view  of  the  position  and  re- 
striction of  the  crystalline  areas.  Aguilera  °  has  made  a  complete 
refutation  of  the  idea  of  this  so-called  Archean  Cordillera,  which, 
even  if  it  has  existed  in  epochs  very  remote,  there  is  no  occasion  to 
consider  iioav  as  such,  since  there  is  so  slight  a  relief  to  these  crystal- 
line schists. 
Whatever  may  have  been  the  origin  assumed  for  these  masses  of 
crystalline  rocks,  and  whether  or  not  they  form  a  basement  upon 
which  rest  the  newer  sediments  and  the  volcanic  rock,  an  important 
question  confronts  us,  namely,  whether  these  crystalline  rocks  really 
form  a  continuous  belt,  elevated  above  the  sea  level  and  only  hidden 
by  the  later  sediments  and  the  volcanic  eruptives,  or  whether,  as 
they  appear  to-day,  they  form  islands,  more  or  less  separated,  pro- 
jecting through  the  other  formations.  We  are  naturally  inclined  to 
believe,  (as  far  as  it  is  possible  to  arrive  at  such  facts)  that  the 
crystalline  rocks  constituted  primarily  a  continuous  belt,  which  has 
been  subjected  in  all  its  parts  to  innumerable  changes — foldings, 
elevations,  depressions,  intrusions  of  foreign  rocks,  etc. — and  that 
little  by  little,  through  the  eifect  of  the  same  forces  which  have 
modified  them,  the  crystalline  belt  has  been  broken  and  the  separate 
blocks  have  been  moved  independently.  The  numerous  prominences 
thus  caused  in  the  belt  have  had  the  effect  of  giving  the  exposed 
parts  of  the  formation  the  character  of  an  archipelago,  or  a  long 
cordon  of  islands,  formerly  much  more  numerous  and  extensive  than 
those  which  Ave  now  discover,  because  considerable  portions  of  the 
formation  have  been  hidden  by  the  later  rocks  of  varying  depths. 
In  the  ocean  depths  an  actual  continuity  probably  exists.  This  is 
probable,  not  on  the  a  priori  assumption  that  these  Archean  rocks  are 
the  foundation  upon  which  rest  the  later  formations  (they  must  have 
some  support),  but  because  these  younger  formations  rise  enormously, 
building  the  Sierras,  supported  by  the  Archean,  thus  justifying  the 
assumption  that  beyond  the  narrow  littoral  platform,  which  extends 
some  tens  of  kilometers,  these  rocks  are  submerged  to  the  bottom  of 
the  steep-shored  depression  of  more  than  a  thousand  fathoms  which 
extends  along  the  Mexican-Pacific  coast  regions. 
The  determination  of  the  early  extension  of  the  belt  of  crystalline 
rocks  is  very  important,  for  it  is  the  basis  of  the  general  tectonic  study 
"  Aguilera,  .1.  <:.,  Sobre  Ins  condicionea  tecttaicas  <!<■  la  Republics  mexicana,  Tip.  Sec. 
Pomento,  L901,  pp.  20-21. 
