18  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1005. 
In  continuation  of  the  work  already  commenced  in  Clear  Creek  and  Gilpin  counties,  Mr. 
Spurr  was  able  to  spend  ten  days,  and  his  assistant,  Mr.  G.  II.  Garrey,  spent  six  weeks 
in  studying  the  mines  in  the  vicinity  of  Idaho  Springs.  A  preliminary  report  by  Mr.  Garrey 
on  the  results  of  this  work  will  be  found  later  in  this  volume. 
In  western  Colorado,  as  part  of  a  reconnaissance  made  of  the  region  south  of  the  La  Sal 
Mountains  by  Mr.  Cross's  party,  Mr.  W.  H.  Emmons  examined  some  interesting  copper 
deposits  in  veins  traversing  Mesozoic  rocks,  and  contributes  a  report  thereon  to  the  pres- 
ent volume.  The  same  geologist  presents  an  abstract  of  his  report  on  the  mines  of  the 
Needle  Mountain  quadrangle,  in  the  southern  San  Juan  Mountains. 
Idaho. — In  Idaho  considerable  reconnaissance  work  has  been  done  in  the  region  both  to 
the  north  and  to  the  south  of  the  Coeur  d'AIene  district,  the  field  work  of  which  was  com- 
pleted by  Mr.  Ransome  and  his  assistants  during  the  field  season  of  1904. 
Mr.  D.  F.  MacDonald,  former  assistant  of  Mr.  Ransome,  made  an  extensive  geological 
reconnaissance  of  the  country  stretching  for  a  hundred  miles  north  of  the  Cceur  d'AIene, 
including  portions  of  northern  Idaho  and  northwestern  Montana,  and  submits  in  the 
present  volume  a  report  on  his  observations  with  special  referen.  e  to  I  heir  economic  bearing. 
Mr.  Collier,  who  was  for  several  years  previous  engaged  in  geological  work  in  the  Alaskan 
division,  also  submits  a  report  on  the  metallic  resources  of  the  region  immediately  adjoining 
the  Ccrur  d'AIene  district  on  the  south. 
Montana. — Late  in  the  year  Mr.  Weed  made  a  visit  to  Butte,  Mont.,  to  study  the  impor- 
tant new  developments  recently  made  in  the  copper  veins  of  this  great  district,  which 
thus  completes  his  record  for  the  exhaustive  report  which  he  is  preparing  on  its  copper 
deposits. 
Nevada. — The  attention  of  the  mining  public  has  been  for  the  past  few  years  especially 
directed  to  Nevada  because  of  the  unusually  rich  gold  ores  found  in  the  newly  discovered 
districts,  such  as  Tonopah,  Goldfield,  Bullfrog,  and  others.  A  proportionately  larger- 
portion  of  the  economic  work  of  the  Survey  has,  therefore,  been  done  in  this  State  to  meet 
the  demand  for  information  with  regard  to  its  geological  structure.  Special  topographic 
maps  had  already  been  prepared  of  the  Goldfield  and  Bullfrog  districts,  and  Mr.  Spurr, 
who  had  made  the  survey  of  the  Tonopah  district,  was  originally  assigned  to  the  economic 
examinations  of  these  districts.  After  Mr.  Spurr's  resignation  lion:  the  Survey  Mr. 
Ransome  took  charge  of  the  work,  having  as  assistants  Mr.  G.  H.  Garrey,  who  had 
previously  been  working  with  Mr.  Spurr,  and  Mr.  W.  H.  Emmons.  He  was,  however, 
unable  to  take  the  field  until  September,  and  owing  to  delays  incident  on  the  unexpected 
difficulty  of  the  work  and  the  unusually  severe  winter  weather  the  field  work,  which  has 
been  made  to  include  also  a  reconnaissance  of  the  Searchlight  district,  is  only  just  com- 
pleted— too  late  to  admit  of  the  preparation  of  any  preliminary  report  for  (his  volume. 
During  the  past  field  season  a  party  under  Mr.  R.  II.  Chapman  has  been  engaged  in  mak- 
ing a  topographical  survey  of  the  portions  of  the  Great  Basin  region  in  southern  Nevada 
and  eastern  California  that  include  the  new  mining  developments.  To  this  party  Mr. 
S.  H.  Ball  was  attached,  as  geologist,  to  make  such  geological  surveys  of  the  region  as 
was  possible  during  the  relatively  rapid  movements  of  the  topographic  party.  In  the 
course  of  this  work  he  wTas  enabled  to  visit  a  number  of  smaller  mining  districts  and  note 
i  heir  geological  structure  and  such  facts  with  regard  to  their  ore  deposits  as  could  be  ascer- 
tained at  the  present  stage  of  their  development.  His  notes  on  these  districts,  although 
the  geological  information  they  afford  is  necessarily  somewhat  meager,  are  published 
in  this  volume,  in  response  to  the  popular  demand  for  early  information  with  regard 
to  new  districts.  It  must  be  borne  in  mind,  however,  by  those  who  have  made  these 
demands,  that  the  geologist  can  not  see  any  farther  into  a  rock  than  any  other  man;  he  can 
only  draw  inferences  from  what  is  exposed  to  view,  and  the  deeper  I  he  exposure  the  greater 
its  value  for  such  inferences,  since  surface  alteration  of  the  rocks  so  obscures  their  true 
character  as  to  render  their  determination  very  uncertain.  As  this  surface  alteration 
often  extends  for  a  considerable  depth  below  the  actual  surface,  it  is  only  when  he  can 
extend  his  studies  to  a  depth  that  is  beyond  the  reach  of  surface  alteration  that  the  geologist 
