THE   CORDILLERAS.  775 
also  numerous  ones  of  quartzite.  This  must  mean  that  somewhere 
is  a  pre-Unkar  sedimentary  series  from  which  the  quartzite  was  de- 
rived. No  such  quartzite  is  known  nearer  than  the  Uncompahgre 
Mountains  of  southwestern  Colorado.  In  Red  Canyon  intrusive  dia- 
base or  basalt  was  observed  in  the  lower  portion  of  the  Unkar  terrane. 
The  Unkar  series  rests  upon  the  truncated  edges  of  the  Archean  in 
its  typical  development. 
The  Archean,  notwithstanding  the  great  variety  of  rocks  which 
compose  it,  seems  to  have  been  almost  absolutely  base-leveled,  at 
least  for  the  district  where  we  saw  it.  The  contact  between  the  con- 
glomerate and  the  Archean  is  regular,  as  the  bedding  of  the  Unkar 
seems  to  be  absolutely  parallel  with  it.  Various  measurements  of 
the  dip  of  the  contact  plane  of  the  Unkar  beds  were  taken,  the  results 
ranging  from  10°  to  12°  NE.  The  descriptions  written  for  the  Lake 
Superior  Archean  in  Bulletin  86  will  apply  exactly  to  the  Archean  of 
the  Grand  Canyon.  Black  hornblende  gneiss  and  light-colored 
gneissoid  granite  are  abundant.  In  many  cases- these  are  inter- 
laminated.  In  some  places  the  gneisses  are  distinctly  banded;  in 
others  they  are  distinctly  foliated  or  schistose.  The  gneisses  are  cut 
by  various  granites  in  the  most  complex  manner,  and  great  exposures 
show  the  dikes  ramifying  and  coming  together  in  the  most  fantastic 
manner.  This  indicates  that  before  the  granites  were  intruded  the 
old  gneisses  had  been  most  completely  deformed.  None  of  the  granite 
dikes  penetrate  the  Unkar.  Dark-colored  gneisses  seem  to  be  the 
oldest  rocks  of  the  series. 
The  great  Grand  Canyon  group,  as  a  result  of  its  gentle  dip  to  the 
northeast  of  10°  or  12°,  has  had  its  entire  thickness  truncated.  At 
one  place  the  two  unconformable  planes  already  described  come  to- 
gether at  an  angle  of  10°  or  12°,  the  inclined  plane  being  that  between 
the  Unkar  and  the  Archean.  It  is  perfectly  clear  Umt  the  Archean 
in  all  its  complex  parts  and  the  Unkar  have  behaved  as  a  unit  in  the 
deformation,  being  tilted  together  at  the  gentle  angle  described. 
Both  old  and  new,  weak  and  strong,  simple  and  complex,  have  been 
gently  inclined  10°  or  12°,  with  comparatively  little  differential 
movements  within  the  mass. 
Resting  unconformably  upon  the  Archean  and  the  Unkar  is  the 
Tonto  sandstone.  The  latter  is  horizontal,  and  is  seen  for  miles  to 
lie  upon  the  same  uneven  surface  of  the  two  previous  formations. 
The  plane  between  the  Tonto  and  the  underlying  formations  has  not 
the  same  marvelous  evenness  as  that  between  the  Unkar  and  the 
Archean. 
Blake,25  in  1899,  refers  to  the  Archean  the  thick  layers  of  gneiss 
forming  the  southern  Hank  of  the  Santa  Catalina  Mountains  in  Ari 
zona.     The  gneiss  is  in  Hat  layers  representing  beds.      A  pari  of  it  i^ 
augen  gneiss;  other  layers  are  quartzose  and  seemingly  quartzites. 
