762  PRE-CAMBRIAN    GEOLOGY    OF    NORTH   AMERICA. 
of  granite  are  very  ancient,  forming  for  the  most  part  large  intrusions 
and  dikes.  In  this  region  there  appears  to  be  more  variety  in  the 
granite  rocks  than  in  the  crystalline  schists,  which  are  reducible  prop- 
erly to  two  types,  mica  schist  and  gneiss.  The  real  extension  of  the 
so-called  Archean  formation  of  the  central  part  of  the  California 
Peninsula  is  unknown  to  us,  but  judging  from  the  studies  of  Emmons 
and  Merrill a  to  the  north  and  south  of  parallel  30°  the  granitic  rocks 
and  the  metamorphic  slates  continue  in  the  sierra  which  follows  al- 
most parallel  to  the  gulf  coast. 
In  the  southern  region  of  the  peninsula,  near  the  cape,  Eisen  b 
marks  on  his  geological  map  a  considerable  outcrop  of  granite  rocks, 
and  Castillo,0  in  his  geological  map  of  Triunfo,  indicates  gneiss  and 
mica  schists  which  he  considered  as  very  ancient.  Emmons  and  Mer- 
rill do  not  name  any  age  for  these  metamorphic  rocks  north  of  par- 
allel 31°,  but  if  the  sierra  which  they  form  is  a  prolonged  region  of 
that  of  Calamahi  we  would  venture  to  declare  them  Archean,  very 
much  eroded  and  cut  by  numerous  granite  intrusions  and  other  rocks 
of  a  later  age. 
It  is  possible  that  the  little  patches  of  metamorphic  rock  which 
Lindgren  d  describes  in  the  regions  to  the  east  of  Todos  Santos,  where 
he  found  them  very  much  cut  up  by  granite,  may  be  Archean. 
SONORA    AND    SINALOA. 
Passing  now  to  the  State  of  Sonora,  Aguilera  e  found  Archean 
rocks  principally  in  the  district  of  Hermosillo  and  in  the  district  of 
Altar,  likewise  cut  by  numerous  granite  masses.  Dumble  f  has  seen 
mica  slates  with  bands  of  more  quartzose  rocks  in  hills  not  very  far 
to  the  north  of  Alamos,  which  are  considered  by  him  as  probably 
Archean.  It  has  been  questioned  whether  these  slates,  referred  to  by 
Dumble,  are  so  ancient. 
In  Sonora  there  are  many  regions  where  there  exist  in  small  patches 
crystalline  schists  and  granites,  pegmatites,  diorites,  etc.,  of  an  age 
much  later  than  the  schists  above  referred  to.  The  supposed  Archean 
areas  of  the  districts  in  the  north  of  the  State  of  Sinaloa  are  little 
known  to  us,  so  that  we  can  not  express  any  idea  as  to  their  petrog- 
raphy or  extension. 
a  Geological  sketch  of  Lower  California  :  Bull.  Geol.  Soc.  America,  vol.  5,  1804.  The 
profile  given  by  these  authors  shows  very  clearly  the  importance  of  having  there  the 
metamorphic  rocks. 
&  Explorations  in  the  cape  region  of  Lower  California  in  1894  :  Proc.  California  Acad. 
Sci.,  2d  ser.,  vol.  5,  1895. 
c  Castillo,  A.  del,  Piano  geologico  y  minero  del  Triunfo  y  San  Antonio,  revisado  en 
1889. 
d  Lindgren,  Waldemar,  Notes  on  the  geology  of  Lower  California,  Mexico  :  Proc.  Cali- 
fornia Acad,  sci.,  2d  ser.,  vol.   1,  1888. 
e  Aguilera,  J.  G.,  Bosquejo  geologico  de  Mexico  :  Bol.  Inst.  geol.  Mexico,  Nos.  4,  5,  6, 
1896,  p.  194. 
'  Notes  en  the  geology  of  Sonora,  Mexico  :  Trans.  Am.  Inst.  Min.  Eng.,  vol.  29,  1899. 
