210  CONTRIBUTIONS    TO   ECONOMIC    GEOLOGY,  1906,  PART    I. 
The  iron  ore  is  younger  than  the  anorthosite,  but  since  each  is  mas- 
sive no  orogenic  movements  of  importance  separated  the  two  intru- 
sions. The  writer  considers  the  iron  ore  and  the  anorthosite  differ- 
entiation products  of  a  common  magma,  the  iron  ore  having  been 
intruded  into  the  anorthosite  after  that  rock  had  completely  solidified. 
The  relationship  of  the  two  rocks  is  shown  not  only  by  their  close  ] 
association,  but  also  by  the  presence  in  each  of  similar  minerals,  the 
two  differing  in  the  proportion  rather  than  in  the  kind  of  minerals 
composing  them.  Iron  ore  is  locally  abundant  in  the  anorthosite 
near  the  iron-ore  lens,  while  feldspar  occurs  sparingly  in  the  iron  ore, 
and  biotite  and  olivine  are  present  in  each.  Chemically  the  mono- 
clinic  pyroxene  of  the  anorthosite  and  the  monoclinic  hornblende  of 
the  iron  ore  probably  approximately  balance  one  another.  Apatite, 
present  in  the  anorthosite,  was  not  observed  in  the  iron  ore,  although 
the  phosphorus  determined  in  two  analyses  of  the  ore  probably  indi- 
cates its  presence.  The  spinel  of  the  ore"  is,  then,  the  only  mineral  not 
common  to  both  rocks.  Spinel  has,  however,  been  found  in  rocks 
rather  similar  to  the  anorthosite  in  composition  from  other  regions. 
The  loose  masses  of  iron  ore  broken  off  from  the  main  mass  have 
already  been  mentioned.  Chugwater  Creek  has  carried  these  down- 
stream and  rounded  them,  and  it  was  these  rounded-ore  pebbles  that 
first  attracted  the  notice  of  the  early  explorers.  Where  the  Colorado 
and  Southern  Railroad  crosses  the  creek,  10  miles  from  the  iron-ore 
body,  the  pebbles  are  1  inch  in  diameter  and  beautifully  rounded. 
Much  larger  pebbles  of  iron  ore,  some  6  inches  in  diameter,  occur  in  the 
Pleistocene  terrace  deposits  that  characterize  this  portion  of  the  front 
of  the  Rocky  Mountains.  The  difference  in  the  size  of  the  bowlders 
in  1  he  t  wo  stream  deposits  rudely  indicates  the  differences  between  the 
transporting  power  of  the  stream  depositing  them. 
Apparently  the  youngest  of  the  pre-Cambrian  rocks  is  a  granite. 
This  certainly  occurs  in  dikes  in  the  anorthosite  and  probably  cuts  the  I 
iron  ore  as  well.  It  is  a  pinkish-gray,  medium-grained  biotite  granite, 
some  specimens  of  which  contain  pink  porphyritic  feldspars  one-half 
inch  long.  It  grades  into  and  is  cut  by  a  biotite  pegmatite.  Magne- 
tite is  at  some  places  present  in  the  pegmatite,  and  at  such  places  the 
magnetite,  which  distinctly  belongs  to  the  pegmatite  period  of  intru- 
sion, impregnates  the  immediately  surrounding  granite.  Under  the 
microscope  the  granite  is  seen  to  be  an  even  granular  rock  rich  in 
microcline  and,  for  a  granite,  rather  poor  in  quartz. 
The  presence  of  an  iron  ore  as  a  visible  constituent  of  the  three1 
igneous  rocks  of  this  area  and  as  the  dominant  constituent  of  one  of 
them  is  worthy  of  note.  This,  then,  may  be  considered  a  pre-Cam- 
brian metallographic  province,"  characterized  by  abundant  magnetite. 
oSpurr,  J.  E.,  Prof.  Paper  U.  S.  Geol.  Survey  No.  42,  1905,  p.  276. 
