764  PRE-CAMBRIAN    GEOLOGY    OF    NORTH   AMERICA. 
Here  the  crystalline  schists,  although  covering  a  very  large  zone, 
are  not  much  varied,  since  most  of  them  may  be  classed  as  biotite 
gneiss,  which  passes  to  granite  and  very  rarely  in  places  to  mica 
schist. 
The  Archean  schists  of  Acapulco  show  similar  simplicity.  These 
we  have  examined  and  described  in  a  study,  unfortunately  not  pub- 
lished, undertaken  some  years  ago  by  the  Institute  of  Geology,0  from 
which  we  extract  some  paragraphs : 
The  studies  made  on  the  interoceanic  profile  between  Acapulco  and 
Vera  Cruz  gave  us  the  opportunity  to  examine  with  considerable  de- 
tail a  line  practically  transverse  to  the  Sierra  Madre  del  Sur — that 
is  to  say,  in  a  direction  southwest-northeast,  thus  obtaining  a  good 
idea  of  the  structure  of  this  sierra. 
As  we  have  already  said,  the  Archean  rocks  constitute  part  of  the 
basement  of  this  sierra  and  are  crowned  throughout  by  more  recent 
rocks — in  some  places  Mesozoic  sediments  and  in  other  places  eruptive 
Tertiary  rocks.  There  are  likewise  ancient  granite  ma'sses,  which  are 
in  evidence  particularly  in  the  structure  of  the  high  mountain 
ridges.  This  granite,  as  outlying  abutments,  is  found  very  close 
to  the  sea,  changing  somewhat  the  general  physiography  of  this 
sierra.  There  is,  indeed,  a  very  marked  contrast  between  the  series 
of  low  hills  formed  of  Archean  schists  which  border  the  sandy 
beaches  or  the  base-level  of  the  coast  and  the  high  mountain  ridges 
of  granite  which  are  raised  here  and  there,  breaking  the  general 
real  elevation  of  the  crystalline  belt  at  the  foot  of  the  sierra. 
The  Bay  of  Acapulco,  in  the  shape  of  a  bottle,  is  formed  by  the 
high  mountain  ridges  of  granite,  lapped  by  the  waters  of  the  sea  on 
the  outer  sloping  sides  which  surround  it. 
At  the  entrance  of  the  Bay  of  Acapulco  the  point  of  the  mountain 
of  Caravali,  with  a  height  of  900  meters,  towers  above  the  surround- 
ing circle  of  mountains.  Linked  to  this  mountain  is  a  chain  of  granite 
and  diorite  ridges  which  advance  to  the  summit  of  the  Sierra  Madre 
del  Sur,  inclosing  within  themselves  different  lateral  valleys  in  the 
form  of  a  horseshoe,  very  similar  in  contour  to  the  Bay  of  Acapulco, 
which  has  suggested  to  us  the  idea  of  considering  the  bay  as  a  sub- 
merged valley,  which  indeed  can  be  proved  by  the  study  of  the  con- 
tour lines  below  the  waters  of  the  ocean. 
Submerged  valleys  similar  to  that  of  Acapulco  have  been  observed 
along  the  Pacific  shores,  demonstrating  that  there  has  probably  been 
a  general  submergence  the  whole  length  of  this  coast.  It  is  possible 
to  prove  movements  of  actual  submersion  in  various  parts  of  the 
coast  of  southern  Mexico. 
a  Aguilera,  J.  G.,  Boese,  E.,  and  Ordonez,  E.,  Perfil  geolSgico  de  Acapulco  a  Veracruz, 
1900. 
