328  PRE-CAMBRIAN    ROCKS    OF    NORTH   AMERICA.  [bull.  86. 
seen  in  the  Virgin  range,  and  in  the  next  ridge  to  the  west,  through  which 
the  river  has  cnt  Bowlder  canyon,  are  gneisses  so  similar  to  those  of  the 
Virgin  range  that  they  may  safely  be  classed  with  them.  In  Mnsic  moun- 
tain, in  the  Black  hills  near  Prescott,  and  on  Canyon  creek,  or,  more  gener- 
ally, all  along  the  soutWestern  border  of  the  Plateau  region  in  Arizona, 
the  Archean  schists  and  granites  are  seen  beneath  nonconforming  mem- 
bers of  the  fossiliferous  rocks,  usually  the  Tonto  sandstone.  To  the 
south  and  west  of  this  line  stretches  a  great  ocean  of  metamorphic  ridges 
in  which  no  one  has  found  fossils.  Whether  a  portion  of  the  rocks  are 
altered  Paleozoic  or  whether  the  Paleozoic  has  been  completely  re- 
moved in  the  progress  of  erosion,  or  whether  the  Archean  rocks  have 
been  covered  by  no  later  ocean  sediments  has  not  been  decided.  The 
purity  and  great  thickness  of  the  Carboniferous  limestones  up  to  the 
very  margin  of  the  region  would  appear  to  negative  the  idea  of  a  perma- 
nent continent  from  Archean  time,  if,  indeed,  it  is  not  negatived  by  the 
survival  of  acute  mountain  ridges. 
Gilbert,70  in  1875,  describes  the  range  region  of  western  New 
Mexico  and  eastern  Arizona.  Northwest  of  the  Burro  mountains  for 
50  miles  are  islands  of  Archean  and  Paleozoic  rocks.  The  most  con- 
spicuous of  the  former  is  a  deep-red  granite.  In  the  Santa  Eita  moun- 
tains the  axial  rocks  are  Archean  schists.  On  the  eastern  border  of 
the  Plateau  region  is  a  chain  of  ranges  which  coalesces  with  the  Rocky 
mountains  of  Colorado  and  consists  mainly  of  Archean  and  Carbonif- 
erous rocks.  The  whole  front  of  the  Sandia  mountains  except  the  crest 
is  Archean.  The  Zufii  range  of  the  Plateau  region  has  a  crystalline 
nucleus  which  Howell  suspects  to  be  due  to  the  metamorphisni  of 
lower  Paleozoic  strata,  as  they  are  conformable  with  unaltered  upper 
Paleozoic  beds.  The  specimens  show  a  gradation  from  compact  sand- 
stones to  gneissic  quartzite  and  quartzose  granite.  Between  the 
Archean  and  the  Silurian  there  is,  first,  a  wide  unconformity,  demon- 
strating the  tilting  and  erosion  of  the  Archean  beds  anterior  to  the 
deposition  of  the  Silurian;  and,  second,  there  is  always  at  the  contact 
a  contrast  of  conditions  as  regards  metamorphism,  the  Silurian  rocks 
being  usually  merely  indurated  and  the  Archean  invariably  highly 
metamorphic.  The  two  characters  of  the  break  serve  to  show  that  it 
represents  a  vast  chasm  of  time,  a  chasm  the  duration  of  which  may 
have  been  greater  than  that  of  the  ages  which  have  since  elapsed.  A 
third  character  of  the  break,  one  that  is  supported  by  less  evidence 
but  is  negatived  by  none,  is  that  the  lowest  of  the  superposed  rocks 
are  conglomerates  and  coarse  sandstones.  The  conclusion  to  be  drawn 
from  the  coarse,  fragmental  nature  of  the  lower  deposits  is  that  the 
water  which  spread  them  was  an  encroaching  ocean,  rising  to  possess 
land  that  had  long  been  dry.  The  recognized  interpretation  of  a  wide- 
spread sandstone  is  continental  submergence,  or,  what  is  the  same 
thing,  an  advancing  coast  line. 
Marvine,37  in  1875,  states  that  granite  is  found  below  the  Tonto 
