200  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1905. 
of  the  igneous  rocks  can  not  now  be  stated.  They  may  have  contributed  hot  waters  bear- 
ing the  iron  ore;  they  may  have  contributed  hot  waters  which  have  leached  the  ore  from 
the  intruded  limestones;  they  may  have  contributed  only  heat  necessary  to  make  meteoric 
waters  highly  effective  in  concentration;  or  probably  there  was  some  combination  of  these 
factors. 
The  ores  are  predominantly  magnetic,  particularly  in  their  upper  portions.  Locally, 
as  in  Utah,  they  grade  down  into  hematite.  The  magnetite  sometimes  shows  a  thin  sur- 
face alteration  to  limonite.  In  places  the  ore  is  largely  limonite.  The  iron  content  locally 
runs  up  to  65  or  06  per  cent,  but  the  average  is  lower.  A  considerable  proportion  of  the 
ores  may  average  above  50  per  cent.  The  phosphorus  content  is  variable  hut  usually 
high.  The  phosphorus  often  occurs  in  apatite  in  large  crystals,  making  it  possible  to  ell'eet 
a  rude  separation  of  high  phosphorus  ores  by  hand  sorting.  A  considerable  proportion 
of  the  ores  contain  iron  or  copper  sulphides  or  both  to  such  an  extent  as  to  require  roasting, 
though  to  this  there  are  important  exceptions. 
The  exposures  are  such  that  the  horizontal  dimensions  of  the  ores  may  be  ascertained. 
The  vertical  extent  down  the  usually  inclined  contact  surfaces  has  for  the  most  part  not 
been  determined.  Depths  of  more  than  H(M)  feet  are  known,  and  the  geological  structure 
makes  greater  depths  probable.  Neither  is  it  known  how  rapidly  the  deposits  narrow 
below.  Until  these  facts  are  known,  satisfactory  estimates  of  tonnage  can  not  be  made. 
While  no  single  western  district  has  yet  been  shown  to  be  comparable  both  in  quantity  and 
quality  of  ores  with  any  one  of  the  Lake  Superior  districts,  it  is  still  certain,  from  the  facts 
now  known,  that  the  Wesl  will  in  the  aggregate  furnish  a  large  supply  of  iron  ore  and  that 
the  expenditure  of  money  for  exploration  and  exploitation  is  warranted.  This  does  not 
mean  that  all  of  the  western  iron  ores  may  become  immediately  available,  for  the  grade 
and  quality  of  the  ores,  together  with  their  distance  from  consuming  centers,  may  make 
it  necessary  to  hold  them  for  many  years. 
