136 
CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.         L bull.  260. 
siliceous  jacket  or  casing,  such  as  is  known  to  surround  ore  bodies  in 
some  other  parts  of  the  world.  While  the  siliceous  casing  may  be  25 
or  30  feet  wide  the  auriferous  portion  may  be  only  1  or  2  feet,  and  the 
form  and  extent  of  this  portion  become  evident  only  after  the  ore  has 
been  extracted.  It  is  then  seen  to  have  a  definite  channel-like  shape, 
often  more  regular  than  that  of  the  whole  outcropping  reef,  though  it 
has  usually  a  limited  extent  in  the  direction  of  its  greatest  elongation 
(fig.  10).  It  seems  that  these  pay  shoots  represent  the  main  channels 
of  hot-water  circulation,  while  the  siliceous  casings  are  the  result  oft 
the  water  soaking  through  the  rock  adjacent. 
Value  <>f  ores. — The  ores  are  often  of  very  high  grade.     As  ari 
extreme  example  may  be  noted  a  shipment  of  14J  tons  from  the  Sand- 
Fig.  10. — Diagrammatic  vertical  cross  sections  of  quartz  bodies,  based  upon  actual  deve 
opment  of  mines  in  Goldfleld.  Dotted  portion  is  quartz  mass  ;  black  streaks  are  oi 
shoots. 
storm  (Kendall  claim),  which  when  worked  in  a  stamp  mill  yielded 
$45,783  net,  while  the  tailings  still  contained  about  $1,000  to  the  toi 
From  the  McKane-Bowes  lease  on  the  Jumbo  there  was  taken  out  i 
five  months,  from  a  space  100  feet  long  and  200  feet  deep  on  the  shoo 
$t>00,000.  One  small  shipment  from  this  lease— 917  pounds  of  ore- 
gave  gross  returns  of  $4,766.  The  whole  production  of  the  cam 
lias  been  from  ore  which  may  be  roughly  estimated  as  averagimi 
$200  to  $300  per  ton  or  more.     The  values  are  generally  all  in  golc 
