THE   CORDILLERAS.  755 
the  base  losing  its  lamination  and  passing  into  a  kind  of  granite. 
(2)  Phyllite  gneiss,  resting  upon  and  grading  below  into  preceding 
beds.  (3)  Very  abundant  mica  schist,  in  some  places  garnetiferous, 
and  in  perfect  conformity  with  the  phyllite  gneisses.  (4)  Phyllites, 
very  argillaceous  in  the  upper  part,  and  toward  the  base  showing 
gradual  diminution  in  the  proportion  of  clay.  In  accordance  with 
this  change  of  composition,  the  structure  varies  from  perfectly 
schistose  to  laminated  and  finally  to  stratiform. 
After  the  deposition  of  the  argillaceous  phyllites,  and  before  the 
termination  of  the  Paleozoic,  there  have  occurred  numerous  eruptions, 
in  order  of  age  as  follows:  Granite  gneiss,  granite,  granulite,  horn- 
blende granite,  pegmatite,  greisen,  and  diorite. 
See  the  following  summary  by  Ordonez  for  references  to  other 
articles  on  the  pre-Cambrian  of  Mexico. 
SUMMARY   OF  PRESENT  KNOWLEDGE  OF   MEXICO. 
The  following  summary  has  been  prepared  by  Senor  Ezequiel 
Ordonez,  formerly  of  the  Mexican  Geological  Survey : 
LOCATION,   IMPORTANCE,    AND   CHARACTER   OF   PRE-CAMBRIAN    AREAS. 
The  areas  which  the  pre-Cambrian  rocks  occupy  in  Mexico  are 
very  small  compared  with  the  very  large  and  uniform  areas  of  Meso- 
zoic,  Tertiary,  and  post-Pleistocene  formations  which  cover  Mexico 
and  define  in  a  general  way  the  different  geographic  zones,  of  which 
the  principal  are  the  eastern,  the  western,  and  the  central. 
The  pre-Cambrian  rocks  are  found  chiefly  in  the  Pacific  coast 
region  of  Mexico.  They  occur  sometimes  in  belts,  more  or  less  nar- 
row, and  again  in  small,  irregular  areas,  the  outlines  of  which  have 
not  been  determined  with  exactness. 
The  most  noticeable  feature  of  these  primitive  formations  is  that 
at  present  in  many  parts  they  do  not  in  any  direct  manner  affeel  the 
physical  relief  of  the  country;  that  is  to  say,  they  appear  entirely 
subordinate  to  the  bolder  relief  shown  by  the  more  recent  Mesozoic 
and  the  eruptive  Tertiary  rocks,  both  of  which  cover  more  than  half 
of  the  country  in  its  most  mountainous  regions. 
Throughout  the  Pacific  coast  zone,  not  considering  Lower  Califor- 
nia, the  pre-Cambrian  rocks  are  found  in  small  areas  irregularly 
scattered  in  the  northwestern  States  of  Mexico,  such  as  Snnora  and 
Sinaloa,  occupying  the  lower  part  of  the  western  flank-  of  the  Sierra 
Madre  Occidental,  or  they  rise  as  low,  isolated  knobs  or  hills  in  the 
coastal  plains  without  any  well-marked  connection  with  the  compo- 
nent orographic  element-  of  the  Sierra  Madre. 
Tn  the  central  part  of  this  Pacific  coast  region,  between  the  paral- 
lels of  latitude  19°  and  24°  north,  there  is  a   wry  marked  interrup- 
