THE   CORDILLERAS.  763 
In  the  State  of  Jalisco,  on  the  western  flanks  of  the  great  sierra, 
which  bounds  to  the  west  the  valley  of  Mascota,  fronting  the  coasts 
of  Tomatlan,  in  the  mining  region  of  Desmoronado,  there  exists  a 
vast  granite  formation  intruded  into  the  crystalline  schistose  mass, 
especially  the  mica  schists,  under  a  heavy  covering  of  quartzite. 
These  rocks  we  have  considered  provisionally  as  Archean. 
GUERRERO. 
Let  us  now  undertake  to  give  a  general  idea  of  the  complex  crystal- 
lines of  the  southern  region  of  Mexico,  where,  as  we  have  said,  the 
Archean  rocks  occupy  a  greater  surface  area.  It  has  been  seen  that 
they  are  here  extended  as  a  belt  or  zone  covering  the  coastal  districts 
of  Guerrero  and  Oaxaca  and  that  they  extend  to  the  southern  part  of 
the  State  of  Puebla. 
We  have  seen  that  the  Archean  band  or  zone  in  Guerrero  begins  in 
a  relatively  narrow  area  in  the  district  of  Galeana,  in  the  State  of 
Guerrero,  to  the  northwest  of  Petatlan,  and  follow-  without  any 
break  along  the  coasts  of  Acapulco,  of  San  Marcos,  and  of  Jamiltepec, 
in  the  State  of  Oaxaca;  it  continues  through  Puerto  Angel  and  the 
Isthmus  of  Tehuantepec,  and  along  the  coasts  of  Chiapas.  In  Ga- 
leana the  zone  of  crystalline  slates  is  less  than  20  kilometers  in  width : 
however,  in  the  district  of  Tabarez,  near  Acapulco,  it  becomes  in 
places  80  kilometers  wide. 
Elsewhere"  we  have  described  briefly  the  crystalline  rocks  of  tin' 
slopes  of  the  Sierra  Madre  del  Sur  near  Tecpam,  about  100  kilometers 
northwest  of  Acapulco.  These  rocks  are  found  up  to  an  altitude  of 
450  meters  above  sea  level,  being  extended  in  a  zone  between  the 
heights  of  the  sierra  formed  of  the  modern  rocks  and  the  recent 
formation  of  the  plains  of  the  coast,  through  which  they  appeal-  in 
the  hills  near  the  seashore,  consisting  principally  of  biotite  gneiss, 
very  easily  exfoliated,  which  passes  below  into  a  granite  gneiss  and 
in  places  almost  to  a  true  granite. 
The  biotite  gneiss  is  changed  rarely  into  muscovite  gneiss,  and  the 
crystalline  rocks  are  crossed  by  dikes,  which  run  in  all  directions. 
These  dikes  are  of  quartz,  of  pegmatites  with  large  crystals  of  rosy 
feldspar,  of  micro-granite,  and  lastly  of  green  dioritic  rocks,  which 
are  the  most  recent. 
As  in  other  places,  the  crystalline  rocks  are  here  broken  by  greai 
masses  of  intrusive  rocks  (granites  and  diorites),  extending  much 
above  the  very  denuded  formation  of  the  -late-. 
"Un  voyage  8  la  Sierra  Madre  dd  Sur:  Mem.  Soc.  cient.  A.  Ai/atr.  vol.  it.  Mexico, 
1899-15)00,   pp.    159-173. 
