THE   CORDILLERAS.  761 
era  in  Mexico,  which  era  has  left  only  insignificant  traces  and  these 
generally  not  well  established ;  but  it  is  possible  also  that  we  shall  not 
have  much  light  in  the  future,  because  in  truth  the  modern  sediments 
and  our  Tertiary  eruptives  have  covered  much  of  our  territory,  so 
that  if  some  day  Paleozoic  areas  should  be  discovered  there  they 
would  necessarily  be  very  small  and  perhaps  insufficient  to  tell  us  the 
condition  of  this  part  of  the  continent  during  the  Paleozoic  era. 
There  is  in  Mexico,  for  the  same  reason,  much  uniformity  of  location 
of  the  formations  actually  known,  and  it  is  not,  then,  to  be  wondered 
at  that  we  must  consider  all  these  evidences  in  interpreting  the 
tectonic  scheme  which  contributes  to  the  general  forms  of  Mexico. 
LOWER    CALIFORNIA. 
According  to  the  conclusions  of  J.  M.  Ramos,a  set  forth  in  a  succinct 
geological  study  of  the  region  of  Calamahi,  north  of  San  Ignacio. 
in  the  central  part  of  Lower  California,  there  exists  near  parallel  29° 
a  relatively  narrow  band  which  this  author  refers  to  the  Primitive 
or  Archean  formation,  this  band  consisting  of  crystalline  schists, 
especially  mica  schists.  Among  the  samples  brought  by  the  explora- 
tion commission  we  have  later  recognized,  in  addition  to  mica  schist, 
biotite  gneiss,  gneissic  granite,  and  some  granites  with  white  mica. 
These  rocks  were  found  in  a  sierra  a  little  distance  from  the  Gulf  of 
California,  which  appears  to  form  a  border  of  a  relatively  high  table- 
land, including  the  valley  of  San  Ignacio.  In  the  center  of  this  valley 
and  on  the  sides  of  a  steep  coastal  sierra,  the  rocky  slopes  of  which 
extend  to  the  waters  of  the  gulf,  Ramos  says  that  the  granites  stand 
out  above  the  schists  as  islands.  These  schists  show  advanced  erosion. 
in  contrast  with  the  masses  of  the  granite,  which  are  of  greater  relief, 
owing  to  their  better  resistance  to  erosion.  Xear  Calamahi,  on  (lie 
mountain  of  Martinica,  Ramos  found  a  series  of  wedges  of  granite 
traversing  the  mica  schists.  The  crystalline  rocks  of  Calamahi  are 
hidden  in  places  under  the  Tertiary  rocks  and  disappear  completely 
toward  the  west,  not  only  beneath  these  rocks  but  chiefly  beneath 
Tertiary  marine  sediments  which  extend  to  the  coasts  of  the  Pacific, 
and  even  to  the  foot  of  the  coastal  sierra  of  the  gulf. 
Everything  goes  to  show  that  a  large  part  of  the  peninsula  was 
submerged  at  the  end  of  the  Tertiary  and  even  at  the  beginning  of 
the  Pleistocene. 
Among  the  crystalline  schists  of  Calamahi  appear  numerous  dikes 
of  pegmatite  with  garnet  and  tourmaline,  micaceous  rocks  (musco- 
vite),  etc. 
Gneissic  biotitic  granite  is  generally  found,  but  we  are  able  to  say 
nothing  in  regard  to  its  position.     It  appear-  to  us  that  the  masses 
"Ramos.  J.  M.,  Informe  de  l<>s  trabajos  ejecutados  por  la  Comisi6n  exploradora  de  la 
Baja  California,  Tip.  Sec.  Fomento,  Mexico,   1886,  p.  76  e1  seq. 
