758  PRE-CAMBRIAN    GEOLOGY    OF    NORTH   AMERICA. 
of  the  whole  country,  since  the  absence  of  Archean.  rocks  in  the  heart 
of  the  Sierra  Maclre  Occidental,  in  the  Mesa  Central,**  and  in  the 
Sierra  Madre  Oriental  argues  much  in  favor  of  the  theory  recently 
advanced  by  Aguilera  and  expressed  very  clearly  in  his  work  on  the 
tectonic  conditions  of  Mexico  b — that  in  the  formation  of  the  geotec- 
tonic  lineaments  of  Mexico  it  is  possible  that  the  crystalline  belt  has 
acted  as  a  zone  of  resistance,  the  general  orientation  of  all  the  moun- 
tain systems  of  the  country  defining  the  aforesaid  belt  and  causing 
along  the  well-marked  curvature  of  the  crystalline  belt  a  region  per- 
petually afflicted  by  active  volcanoes.  It  is  necessary  to  add  that  the 
Archean  massif  has  acted  as  a  resisting  zone,  not  because  it  may  have 
remained  completely  rigid,  since  we  shall  see  that  it  has  been  exposed 
to  numerous  movements,  but  because  it  has  not  been  moved  harmoni- 
ously with  the  mass  of  the  newer  materials,  which  by  their  abundance 
and  characteristic  distribution  are  those  which  give  to  Mexico  its  true 
physiographic  characteristics. 
This  independence  of  movement  gives  sufficient  reason  in  itself  for 
assuming  the  continuity  of  a  single  extended  Archean  mass  lying 
under  the  waters  of  the  sea  and  partly  emerged,  subject  to  forces 
which  have  an  origin  comparatively  deeper. 
We  can  not,  unfortunately,  enter  upon  a  fuller  consideration  of  this 
subject,  but  we  can  point  out  some  conclusions  which  appear  to  follow 
from  the  influence  which  the  Archean  rocks  have  exercised  or  the  ob- 
stacles which  they  have  imposed  to  the  movements  of  the  extensive 
sedimentary  formations.  For  example,  if  there  existed  a  continuous 
crystalline  belt,  more  or  less  broken,  or  it  is  better  to  say  dislocated, 
along  the  Pacific  coast  line,  which  served  as  a  resistant  zone  to  the 
other  rocks,  especially  the  Mesozoic,  or  if  there  was  a  certain  inde- 
pendence of  movements  between  the  one  and  the  other,  it  appears 
probable,  considering  the  actual  position  of  the  Sierra  Madre  Occi- 
dental with  relation  to  these  rocks  and  the  essential  volcanic  nature  of 
the  said  Sierra,  that  this  Sierra  has  been  created  exactly  on  the  con- 
tact of  the  crystalline  rocks  with  the  sedimentary  Mesozoic  rocks,  and 
that  this  contact  is  consequently  a  kind  of  scar  with  Tertiary  volcanic 
rocks  intervening  between  the  Archean  and  Mesozoic  formations.  To 
be  sure,  we  do  not  everywhere  see  the  Mesozoic  rocks  in  immediate 
proximity  with  eruptive  rocks  of  the  Sierra  on  the  side  toward 
the  Mesa  Central,  but  this  is  due  to  the  great  elevation  of  the  mod- 
ern lacustrine  sediments  taking  the  greater  part  of  the  material 
of  which  these  were  formed  away  from  the  volcanic  masses  and  refill- 
«  It  has  been  assumed  by  geologists  like  Humboldt,  Burkart,  and  others,  recently  among 
them  Aguilera  (Bosquejo  geologico  de  Mexico),  that  the  slates  which  are  much  altered 
and  of  an  ancient  aspect,  as  those  at  Guanajuato,  Zacatecas,  Fresnillo,  Catorce,  etc..  are 
Archean,  but  this  remains  to  be  proved.  If  the  existence  of  Archean  rocks  in  the  region 
of  the  Mesa  Central  is  some  day  proved,  that  would  affirm  even  more  strongly  the  impor- 
tance of  these  rocks  in  the  tectonic  history  of  Mexico. 
6  Op.   cit.,    pp.   29-31. 
