760  PRE-CAMBRIAN    GEOLOGY    OF    NORTH   AMERICA. 
laceous  slates,  phyllites,  and  phyllitic  gneisses,  to  which  Aguilera  has 
referred."  These  slates  pass  to  gneiss  and  mica  slates  by  insensible 
transitions,  as  was  indicated  by  the  author  cited.  We  personally  have 
not  seen  these  gneissic  phyllites  in  the  Archean  areas  with  which  we 
are  acquainted,  but  it  is  probable  that  they  are  present,  particularly 
in  the  Archean  areas  farther  in  the  interior — that  is,  in  the  upper  part 
of  all  the  groups — since  it  is  a  curious  thing  that  in  many  places 
toward  the  interior  of  the  country  the  crystalline  strata  of  the  later 
periods  are  found.  At  any  rate,  in  the  most  extensive  Archean  re- 
gions, as  near  Acapulco,  to  the  southwest  of  Oaxaca,  and  in  Tehuan- 
tepec,  are  seen  the  crystalline  schists,  with  very  limited  quantities 
of  phyllites,  not  always  observed  in  conformable  stratification. 
If  we  should  try  to  establish  a  correlation  with  the  large  areas  of 
the  American  continent  our  Archean  in  its  larger  extension  would 
correspond  with  the  Laurentian  period  of  North  America,  because  it 
shows  chiefly  slaty  crystalline  rocks  which  grade  into  the  granite,  as 
in  general  is  set  forth  in  the  work  of  Van  Hise.  However,  in  order 
not  to  introduce  error,  we  do  not  yet  make  any  exact  division  of  the 
Archean  areas  of  Mexico.  This  problem  remains  for  a  later  date, 
when  our  crystalline  series  and  the  few  indisputable  sediments, 
although  metamorphosed,  which  rest  upon  them  may  have  been 
further  studied. 
Of  the  intrusive  granite  rocks  which  appear  through  the  crystalline 
schists,  some  traversed  these  rocks  right  after  the  deposition  or  the  con- 
solidation of  the  primitive  schists,  judging  from  the  evidences  found 
in  them  of  their  having  followed  the  changes  in  the  slate,  as  is  the  case 
in  regard  to  certain  granites  which  pass  insensibly  to  gneiss.  This 
gneissic  granite  is  almost  always  perthitic  and  poor  in  colored 
minerals,  and  is  much  like  some  gneissic  granites  of  the  United  States. 
In  order  to  give  an  idea  of  what  classes  of  rocks  enter  into  the  con- 
stitution of  our  Archean  areas  we  pass  in  review  in  a  few  words  some 
of  the  more  important  of  them.  \Ye  are  far  from  thinking  that  these 
few  lines  give  an  adequate  idea  of  the  crystalline  rocks  of  each  region, 
since  our  sources  of  information  are  very  limited  and  incomplete. 
INSIGNIFICANCE  OF  WELL-KNOWN  PALEOZOIC  SEDIMENTS  IN   MEXICO  AT  PRESENT. 
In  the  first  place  let  us  say  that  it  may  seem  somewhat  exaggerated 
to  assume  that  the  Archean  rocks,  as  ancient  as  they  are,  have  any- 
thing to  do  with  the  movements  of  the  rocks  more  recent,  as  the 
Mesozoic,  or  even  with  the  eruptive  Tertiary  rocks,  in  view  of  the 
fact  that  there  has  intervened  the  enormous  lapse  of  Paleozoic  time, 
unless  these  Archean  rocks  serve  as  an  obstacle.  It  seems  more  pru- 
dent to  leave  these  questions  until  more  is  known  of  the  Paleozoic 
"  Aguilera,  J.  G.,  Bosquejo  geologico  de  Mexico  :  Bol.  Inst.  geol.  Mexico,  Nos.  4,  5,  6, 
1896,   p.   193. 
