310  CONTRIBUTIONS    TO   ECONOMIC    GEOLOGY,  1904.        [bull.  260.  j 
strated  in  so  many  instances  that  it  should  furnish  a  rule  for  future 
exploration.     At  the  same  time  the  actual  agreement  of  the  lower  1 
deposits  with  the  upper  ones  as  to  value,  form,  and  direction  can  not  I 
always  be  relied  upon.     In  selecting  an   area   for  prospecting  care 
should  be  taken  to  select  such  a  one  as  has,  below  the  level  of  ground  I 
water,  a  reasonable  thickness  of  ore-bearing  rock — i.  e.,  of  Galena  I 
limestone,  or,  in  the  eastern  part  of  the  region,  of  at  least  the  upper! 
member  of  the  Platteville  limestone.     While  it  is  possible  that  eco-1 
nomic  deposits  of  value  may  exist  below  the  Platteville  limestone,! 
this  is  not  considered  probable,  and  consequently  expensive  explora-  f 
tions  or  deep  drilling  in  these  formations  is  not  recommended.     At- 1 
tention  is  called  to  the  advisability  of  carrying  on  preliminary  explo- 1 
rations  by  drilling  rather  than  by  sinking  deep  test  pits. and  shafts, 
for  the  expense  of  the  latter  Avork  is  considerably  greater,  and  this! 
expense   is   commonly   increased   by   the   large   quantities   of   water 
encountered.     Experiments  are  uoav  being  made  as  to  the  advisability 
of  using  the  diamond  drill  instead  of  the  ordinary  churn  drill  for] 
prospecting  in  this  district. 
