506  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull.  86. 
PROMO  NTOKY    RIDGE,    ANTELOPE    AND    FREMONT   ISLANDS. 
In  this  range  are  a  series  of  mica-schists,  quartzites,  argillites,  which 
are  occasionally  calcareous,  rocks  which  are  probably  clastic  and  there- 
fore represent  the  Algonkian.  The  relations  of  this  series  to  the  Ar- 
ch ean  complex  are  not  known.  At  Promontory  point  it  rests  uncon- 
formably  under  the  Weber.  The  rocks  on  Antelope  and  Fremont  isl- 
ands are  regarded  as  of  the  same  age  because  of  their  likeness  to  the 
Promontory  point  rocks  and  they  are  believed  to  be  pre-Cambrian 
because  no  Cambrian  or  post-Cambrian  rocks  in  this  region  are  more 
than  indurated; 
THE   AQUI  MOUNTAINS. 
In  the  Aqui  mountains  the  probable  Algonkian  consists  of  a  series 
of  quartzites  6,000  feet  thick  containing  beds  of  conglomerate  with  i 
argillaceous  schists  and  imperfect  mica-schists.     This  series  rests  upon 
the  granite,  winch  is   presumably  a  part  of  the  basal  complex.     It  ifl 
conformably  below  the  basal  Cambrian,  stands  as  Upper  Algonkian,, 
and  may  be  correlated  with  the  Wasatch  Algonkian. 
SCHELL   CREEK,    EGAX,    POGONIP   OR  WHITE    PINE,    AND   PINON   RANGES. 
In  the  Schell  creek  range  the  probable  Algonkian  is  represented  by 
heavy  bodies  of  quartzite.  In  the  Egan  range  the  probable  Algonkian 
is  represented  by  a  series  of  thoroughly  vitrified  quartzites  several 
thousand  feet  thick,  containing  quartzitic  and  micaceous  schists.  Iu 
the  Pogonip  range  it  is  represented  by  micaceous,  arenaceous,  and, 
argillaceous  slates  and  shales  and  by  vitreous  quartzite,  the  series  be.; 
ingof  undetermined  thickness.  In  the  Pifion  range  it  is  represented 
by  quartzites  underlain  by  mica -schists  and  quartzitic  schists  having 
a  total  thickness  of  5,000  feet.  In  all  of  these  ranges  these  series  rest 
nnconformably  upon  the  Archean  and  are  conformably  under  basal^ 
Cambrian.  They  therefore  are  probably  Upper  Algonkian  and  stand 
as  the  equivalent  of  the  Wasatch  Algonkian. 
FRONT  RANGE   OF    COLORADO. 
In  the  district  of  Ralston,  Coal,  Boulder,  and  Thompson  creeks  oi 
the  Front  range  of  Colorado  the  Algonkian  is  represented  by  quartz- 
ites, quartz-schists,  mica-schists,  and  schist-conglomerates,  the  thick- 
ness of  the  series  being  unknown,  but  certainly  more  than  1,000  feet. 
The  series  has  become  very  nearly  crystalline.  Its  structural  relation] 
to  the  basal  complex  are  not  surely  known.  It  appears  to  grade  down- 
ward into  these  rocks,  but  this  may  be  merely  a  superinduced  conform- 
ity. The  series  of  schistose  rocks  in  the  Front  range,  referred  by^ 
King  to  the  Huronian  and  estimated  at  25,000  feet  thick,  probably 
belong  in  large  part  to  the  fundamental  complex. 
