CHAPTER  XIII. 
THE  CORDILLERAS. 
SECTION  1.     MEXICO  AND  CENTRAL  AMERICA. 
SUMMARY   OF  LITERATURE. 
Sapper,1  in  1894,  describes  and  maps  considerable  areas  of  Azoic 
formations  in  Guatemala.  The  lowest  formations  are  gneiss  and  the 
higher  formations  are  mica  schists  and  phyllites,  associated  with 
which  are  crystalline  limestones,  actinolite  schist,  and  quartzites. 
Closely  associated  with  these  schistose  rocks  are  ancient  eruptive 
rocks,  including  granite,  diabase,  etc.  Whether  these  Azoic  forma- 
tions are  pre-Paleozoic  or  not  can  not  as  yet  be  asserted. 
Sapper,2  in  1896,  describes  the  geology  of  Chiapas,  Tabasco,  and  the 
Peninsula  of  Yucatan,  and  mentions  the  occurrence  of  Azoic  rocks 
in  the  Sierra  Madre.  These  rocks  include  gneiss,  mica  slates,  and 
phyllites.  A  band  in  the  first  northern  range  of  the  sierra,  near  the 
plantations  of  Piedad  and  San  Vicente,  trends  N.  70°  W.  and  dips  5° 
NE.  Among  the  bowlders  washed  down  by  Aguacate  River  may  be 
seen  gneiss,  mica  slates,  and  phyllites,  which  indicate  the  presence  of 
the  crystalline  formations  also  in  the  interior  of  the  Sierra  Madre. 
Aguilera,3  in  1897,  gives  a  synopsis  of  the  geology  of  Mexico.  The 
most  ancient,  or  Azoic,  rocks  are  granites,  gneisses,  and  schists,  pre- 
senting many  variations.  They  extend  lengthwise  along  the  Pacific 
coast,  forming  a  narrow  band,  interrupted  in  places,  and  sending 
ramifications  toward  the  central  part  of  the  country,  in  some  places 
almost  to  the  eastern  coast.  They  occupy  the  southern  part  of  the 
State  of  Puebla,  a  part  of  the  Sierra  Madre  in  Chiapas,  and  exten- 
sive portions  of  Oaxaca  and  Guerrero;  they  are  found  also  in  Zapa- 
tecas,  around  Fresnillo;  in  Guanajuato,  in  the  vicinity  of  the  capital; 
in  Sinaloa,  around  the  crests  of  the  Sierra  Madre;  in  Sonora,  in  its 
northwestern  and  western  parts;  in  Lower  California,  wdiere  they  con- 
stitute the  central  Cordilleran  axis  of  the  peninsula,  and  in  Vera- 
cruz, in  its  western  region,  limited  by  Puebla,  in  the  canton  of  Zon- 
golica. 
In  the  southern  part  of  Puebla  and  in  Guerrero  and  Oaxaca,  wThere 
the  greater  part  of  the  exposures  occur,  the  sequence  is  as  follows, 
from  the  base  up:  (1)  Porphyritic  gneiss,  similar  to  augen  gneiss,  at 
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