12  CONTRIBUTIONS   TO   ECONOMIC   GEOLOGY,  1904.        [bull.  2601 
In  the  preparation  of  the  present  volume  promptness  of  publica- 
tion has  been  made  secondary  only  to  the  economic  utility  of  the  ma{ 
terial  presented.  The  papers  included  are  such  only  as  have  a  direct 
economic  bearing,  all  questions  of  purely  scientific  interest  being  ex-* 
eluded. 
The  papers  are  of  two  classes:  (1)  Preliminary  discussions  of  the 
results  of  extended  economic  investigations,  which  will  later  be  pub* 
lished  by  the  Survey  in  more  detailed  form;  (2)  comparatively  de-v 
tailed  descriptions  of  occurrences  of  economic  interest,  noted  by 
geologists  of  the  Survey  in  the  course  of  their  field  work,  but  not  o$ 
sufficient  importance  to  necessitate  a  later  and  more  extended  de- 
scription. A  third  class  of  papers  was  included  in  the  bulletin  fop 
1902,  namely,  abstracts  of  certain  economic  papers  which  had  ap- 
peared in  Survey  publications  during  the  year,  chiefly  such  as  gave 
a  general  account  of  the  distribution  and  mode  of  occurrence  of  par- 
ticular mineral  deposits  throughout  the  United  States.  Most  of  the 
publications  on  economic  geology  which  have  appeared  during  the 
past  year  were  abstracted  for  advance  publication  in  Bulletin  225, 
and  it  has  therefore  been  unnecessary  to  abstract  them  in  this 
volume. 
The  papers  have  been  grouped  according  to  the  subjects  treated. 
At  the  end  of  each  section  is  given  a  list  of  previous  publications  oif 
that  subject  by  this  Survey.  These  lists  will  be  serviceable  to  those 
who  wish  to  ascertain  what  has  been  accomplished  by  the  Survey  ill 
the  investigation  of  any  particular  group  of  mineral  products.  They 
are  generally  confined  to  Survey  publications,  though  a  few  titles  ofl 
important  papers  published  elsewhere  by  members  of  the  Survey  are 
included. 
The  preparation  of  this  bulletin,  as  well  as  of  the  corresponding 
bulletins,  already  referred  to,  has  been  chiefly  the  work  of  Mr.  E.  C. 
Eckel,  to  whom  is  due  in  large  measure  the  credit  for  planning  the 
work  and  carrying  it  to  a  successful  issue. 
The  results  of  the  Survey  work  in  economic  geology  have  been  pub- 
lished in  a  number  of  different  forms,  which  are  here  briefly  described: 
1.  Papers  and  reports  accompanying  the  Annual  Report  of  the 
Director,  United  States  Geological  Survey. — Prior  to  1902  many  eco- 
nomic reports  were  published  in  the  royal  octavo  cloth-bound  volumes 
which  accompanied  the  Annual  Report  of  the  Director.  This  form  of, 
publication  for  scientific  papers  has  been  discontinued  and  a  new 
series,  termed  Professional  Papers,  has  been  substituted. 
2.  Bulletins  of  the  United  States  Geological  Survey. — The  bulletins 
of  the  Survey  comprise  a  series  of  paper-covered  octavo  volumes,  each 
containing  usually  a  single  report  or  paper.  These  bulletins,  for- 
merly sold  at  nominal  prices,  are  now  distributed  free  of  charge  to 
those  interested  in  the  special  subject  discussed  in  any  particular 
