Emmons.]  INVESTIGATION    OF    METALLIFEROUS   ORES.  25 
MISSISSIPPI    VALLEY    REGION. 
This  great  region  stands  in  marked  contrast  geologically  with  the 
mining  region  of  the  West  by  the  practical  absence  of  intrusions  of 
igneous  rocks,  and  by  the  relatively  undisturbed  position  of  its  sedi- 
mentary beds.  Certain  areas  have,  however,  proved  of  great  eco- 
lomic  importance  because  of  the  abundant  and  cheaply  worked  ores 
)f  lead  and  zinc  that  are  found  in  them.  The  most  important  of 
^hese  producing  areas  have  been  studied  and  reported  upon  in  previ- 
>us  years.  During  the  last  season  examinations  have  been  made  by 
Mr.  Bain,  assisted  by  Mr.  Ellis  and  others,  of  some  outlying  and 
litherto  undescribed  districts,  and  information  has  also  been  col- 
ected  with  regard  to  the  occurrences  of  copper  ore,  some  of  which 
;eem  to  be  of  economic  importance,  in  the  same  general  area.  Re- 
Dorts  on  these  will  be  found  in  this  volume. 
ROCKY  MOUNTAIN  REGION.0 
Work  of  the  past  summer  in  this  region  will  be  noted  geograph- 
cally,  the  States  being  mentioned  in  alphabetic  order. 
Arizona. — No  systematic  field  work  has  been  done  in  Arizona  dur- 
ng  the  past  season.  Some  topographic  surveying  has  been  done  as  a 
^reparation  for  future  geological  investigations,  and  the  territory 
vas  traversed  by  the  writer  in  his  official  journeyings.  Some  infor- 
nation  obtained  by  him  with  regard  to  copper  deposits  in  the  Colo- 
rado Plateau  region  has  been  used  for  the  text  of  an  article  on  copper 
leposits  in  Permo-Triassic  beds,  which  appears  in  this  volume. 
California. — Economic  surveys  had  been  planned  for  the  three 
'opper  districts  in  Shasta  County,  Cal.,  which  lie  either  within  or  on 
he  borders  of  the  Reading  quadrangle.  As  the  geologists  assigned 
o  this  work  were  unable  to  complete  their  surveys  in  other  regions 
^arly  enough  to  undertake  it,  the  work  has  been  postponed  until 
mother  season. 
Colorado. — The  resurvey  of  the  Cripple  Creek  district,  as  already 
loted,  has  been  completed.  Economic  work  lias  been  commenced  in 
he  oldest  and  most  continuously  productive  region  of  the  State- 
hat  of  Gilpin  and  Clear  Creek  counties.  Two  parties  have  been 
mployed  here  during  the  summer,  one  in  mapping  the  areal  or  sur- 
face geological 'features  of  the  region,  which  are  unusually  compli- 
cated because  of  the  great  age  and  intensely  metamorphosed  character 
f  the  rocks,  the  other  in  studying  the  internal  rock  structure  and  ore 
eposits,  as  shown  by  the  underground  workings  of  the  mines.     The 
a  No  mention  is  made  here  of  economic  work  in  Alaska,  as  the  bulk  of  the  material 
rith  regard  to  that  region,  which  has  hitherto  been  published  in  the  Economic  Geology 
nilletin,  is  this  year  so  great  that  it  has  been  thought  advisable  to  incorporate  it  in  a 
eparate  volume — Bulletin   No.   259. 
