26  '  CONTRIBUTIONS   TO   ECONOMIC   GEOLOGY,  1904.        [bull.  200. 
area  is  so  great  that  it  was  not  expected  to  complete  the  work  in  a 
single  season. 
The  present  volume  contains  a  preliminary  report  on  the  ore  depos- 
its of  the  Georgetown  or  Silver  Plume  district  by  Mr.  J.  E.  Spuria 
This  district,  from  an  economic  point  of  view,  is  long  past  its  zenith, 
and  a  report  upon  it  necessarily  belongs  in  the  category  of  what  has; 
been  called  "  back  numbers  "  by  some  of  the  critics  of  the  Survey,  who-i 
look  only  to  the  immediately  applicable  results  of  its  work.  The  sci- 
entific student  of  ore  deposits  will  find,  however,  upon  a  perusal  of 
Mr.  Spurr's  report,  that  it  fills  a  gap  in  our  knowledge  of  an  impor- 
tant type  of  ore  deposits — the  older  fissure  veins — which  are  by  no 
means  so  simple  in  their  structure  and  maimer  of  formation  as  has 
generally  been  conceived.  To  the  miner,  on  the  other  hand,  it  is. 
evident  that  the  knowledge  obtained  during  these  studies  will  be  of 
great  practical  use,  if  not  in  this  particular  district,  at  any  rate  inj 
others  in  which  he  may  happen  to  be  Avorking  where  the  same  type  on 
deposits  are  developed. 
Economic  work  incidental  to  areal  surveys  was  carried  on  during 
the  summer  by  Mr.  J.  ]).  Irving  in  the  San  Juan  district,  comple-1 
mentary  to  the  surveys  of  the  Ouray  and  Needle  Mountains  quad- 
rangles,  and  a  small  but  interesting  group  of  mines  in  the  Durango 
quadrangle  which  carry  gold  in  association  with  some  amalgam  were.] 
studied  and  reported  upon  by  Mr.  W.  H.  Emmons,  a  recent  acquisi- j 
tion  to  the  corps  of  geologists.     A  brief  visit  was  also  made  by  thej 
writer  to  Leadville  to  keep  in  touch   with  the  recent  underground 
developments  and  incorporate  their  results  in  his  data  for  the  prepa- 
ration of  a  more  detailed  and  accurate  map  of  this  region. 
Idaho. — In  northwestern  Idaho,  Mr.  Eansome,  with  the  assistance 
of  Mr.  Calkins,  completed  the  survey  of  two  15'  quadrangles,  which 
include  the  mines  of  the  Coeur  d'Alene  district.  These  mines  are 
of  great  economic  importance,  furnishing  as  they  do  one-third  of 
the  annual  lead  product  of  the  United  States,  together  with  about 
0,000,000  ounces  of  silver  and  a  little  gold.  Scientifically,  the  veinfi 
are  of  interest  because  of  their  metasomatic  origin  and  their  occur- 
rence in  a  very  old  series  of  rocks  of  great  thickness  that  have 
never  before  been  studied  in  detail.  A  summary  of  the  results  is 
given  in  this  bulletin  by  Mr.  Ransome. 
Nevada. — The  development  of  important  deposits  of  gold-bearing 
ores  in  south-central  Nevada  has  been  actively  conducted  during  the 
year,  and  in  the  new  district  of  Goldfielcl  phenomenally  rich  bodies 
have  been  opened.  These  developments  have  been  materially  aided 
by  the  establishment  of  railroad  connection  with  the  trunk  lines. 
Mr.  Spurr  made  a  reconnaissance  examination  of  both  Tonopah  and 
Goldfield  in  the  autumn  months,  after  the  snowfall  in  the  mountains 
