224  CONTRIBUTIONS    TO   ECONOMIC    GEOLOGY,  1904.        [bull.  260. 
ent  time  iron  sulphides  are  being  deposited  in  large  quantities  in  an 
ooze  at  the  bottom  of  the  Black  Sea;  also,  on  the  occurrence  of  erup- 
tive tufas  in  the  Mansfeld  series  of  beds,  thinking  it  possible  that 
gaseous  emanations,  or  hot  springs  carrying  copper,  might  have  ac- 
companied their  eruption  or  have  hastened  precipitation.  He  differs 
also  with  the  geologists  of  the  Posepny-Beck  school  as  to  the  facts 
in  the  Mansfeld  district,  maintaining  that  it  is  not  true  that  the 
shales  are  richer  in  the  vicinity  of  the  fractures  which  dislocate  them, 
as  they  maintain.  He  says  that  copper  occurs  in  the  very  uniform 
percentage  of  2  per  cent  in  a  thin  bed  immediately  over  the  conglom- 
erate, which  carries  no  lead  or  nickel.  Where  there  are  verticals  or 
faults  these  carry  some  nickel  and  lead,  and  the  bottom  bed  becomes 
barren  in  their  vicinity,  the  copper  occurring  some  little  distance 
above  it  with  the  nickel.  When  there  is  considerable  distance  be- 
tween the  faults,  the  copper  comes  down  again  to  the  lower  layer, 
which,  however,  carries  no  nickel  or  lead. 
One  of  Bergeat's  students  has  recently  published  a  new  study  of  the 
iron  deposits  in  the  Devonian  limestone  of  Nassau,  in  Germany,  which 
have  long  been  held  as  typical  instances  of  metasomatic  replacement. 
In  this  he  maintains  that  previous  geologists,  assuming  this  origin  to 
be  so  well  established  as  to  be  almost  axiomatic,  have  wrongly  read  the 
facts,  and  that  the  ores  are  syngenetic  or  of  sedimentary  origin. 
In  these  discussions  it  seems  to  me  that  writers  should  be  careful 
to  assure  themselves  that  they  start  with  a  common  premise,  and  that 
when  one  speaks  of  a  sedimentary  or  syngenetic  origin  of  a  deposit 
he  means  the  same  thing  as  his  opponent.  Ore  deposits  may  be  de- 
fined as  concentrations  of  useful  metals  into  bodies  rich  and  large 
enough  to  be  worked  at  a  profit,  but  it  is  rare  that  the  process  of 
concentration  has  been  single.  In  most  cases  they  are  the  result  of 
many  and  quite  different  processes,  and  the  question  is,  Which  of 
these  is  the  characteristic  one  by  which  it  should  be  defined?  The 
German  geologists  seem  to  consider  that  if  sedimentation  has  played 
any  part  in  the  formation  of  the  materials  of  a  deposit  it  is  therefore 
to  be  considered  sedimentary.  I  believe  rather  that  the  process  that 
has  brought  it  into  its  latest  phase  as  a  workable  deposit  is  the  one 
that  should  characterize  it,  and  hence  can  conceive  the  possibility  that 
in  the  case  cited  above,  of  the  Mansfeld  deposits,  each  of  the  opposing 
views  may  be  partially  right.  The  copper  salts  might  have  been 
originally  disseminated  through  the  general  series  of  sediments  and 
later  gathered  together  into  a  concentrated  form  in  a  disturbed 
region  as  a  result  of  conditions  accompanying  the  disturbance. 
This,  for  instance,  is  the  view  held  by  many  with  regard  to  the 
lead  and  zinc  deposits  of  the  Mississippi  Valley,  which  Beck  in- 
cludes in  his  epigenetic  class,  although  their  materials  are  assumed 
