128  CONTRIBUTIONS    TO    ECONOMY    GEOLOGY,   1903.  tBULL.225, 
that  in  the  Lower  levels  a  mass  of  phonolite  of  a  dark-greenish  color 
has  been  found  closely  associated  with  the  wider  portions  of  the  ore 
zone  NO  definite  foot  wall  or  hanging  wall  to  the  ore  can  be  detected, 
because  it  is  in  many  cases  difficult  to  distinguish  the  mineralized  mate- 
rial from  the  barren  country  rock. 
It  seems  probable  that  the  Homestake  lode,  owing  to  its  mineralize! 
character,  was  a  harder  and  more  resistant  ledge  than  the  surround- 
ing schists  of  the  Algonkian,  and  that  for  this  reason  it  constituted  a 
reef  in  the  old  Cambrian  seas  before  the  sedimentary  rocks  above 
were  deposited.  Tbat  it  was  then  mineralized  and  gold-bearing  is 
proved  by  the  presence  of  gold  in  the  basal  or  lowest  rocks  of  the 
sedimentary  series  which  lie  in  the  isolated  patches  about  the  outcrop 
of  the  ore  body. 
The  ores  of  the  Homestake  belt  taken  as  a  whole  can  not  be  said  to 
present  any  constant  features  which  serve  to  distinguish  them  from  the 
characteristic  but  barren  rocks  of  the  Algonkian  series.     Pyrite  is  by 
far  the  most  invariable  indication  of  mineralization,  but  it  is  notably 
absent  from  much  of  the  ore,     Quartz  also  occurs  in  a  great  number 
of  cases.     Perhaps  the  most  usual  type  of  ore  would  be  that  consisting 
largely  of  quartz  and  pyrite.     Other  minerals  are  dolomite,  calcitl 
and  arsenopy rite;  these  are  also  of  very  frequent  occurrence,  but  no 
decrease  in  the  values  of  the  ore  can  be  noted  when  they  are  absent. 
Again,  garnet  and  tremolite  appear  in  some  portions  of  the  ore  in 
sudi  abundance  as  to  constitute  the  larger  part  of  the  gangue,  but  the 
ore  here  found  does  not  differ  in  value  from  that  having  a  wholly 
different  appearance.     It  would  seem  that  when  the  ordinary  type  oi 
schists  is  mineralized  the  ore  more  closely  resembles  a  schist,  hi 
when  the  amphibolite  is  mineralized  the  ore  more  closely  resemble! 
an  amphibolite.     Thus  it  will   appear  that  although  pyrite,  quart! 
dolomite,  calcite,  arsenopyrite,  tremolite,  and  garnet  frequently  con 
stitute,  either  separately  or  in  combination,  the  gangue  of  the  ore,  nc 
one  of  these  minerals  can  be  considered  an  indication  of  the  presence 
of  gold.     In  general,  however,  it  may  be  said  that  the  ores  occupy  i 
zone  in  the  Algonkian  rocks  which  presents  a  greater   number  o 
secondary  minerals,  a  more  constant  occurrence  of  sulphides,  quartz 
dolomite,   calcite,  and   arsenopyrite,   and,   finally,  a   more   advance' 
degree  of  distortion  and  irregularity  of  structure  than  do  the  barre 
areas  of  the  same  formation. 
From  a  careful  study  of  the  ores  and  the  general  structure  of  th| 
Homestake  belt,  it  appears  first,  that  there  have  been  several  differen 
periods  of  mineralization,  at  least  one  of  which  has  preceded  th 
deposition  of  the  Cambrian  rocks,  as  is  distinctly  shown  by  th 
presence  of  placer  gold  in  the  lowermost  gravel  beds  of  the  Cambria 
series  second,  that  there  have  been  periods  of  mineralization  whic 
followed  the  entire  deposition  of    the  sedimentary   rocks  and  wer 
