8  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1906,  PART   I. 
This  change  in  Survey  organization  has  been  used  as  a  basis  for  a 
separation  of  the  economic  bulletin,  based  on  subjects.  The  present 
bulletin  (No.  315)  therefore  covers  the  work  of  the  Survey  in  1906  in 
the  metals,  structural  materials,  and  other  nonmetals  except  fuels. 
A  separate  bulletin  (No.  316)  will  be  issued  later  covering  Survey 
work  on  coal,  lignite,  and  peat. 
In  the  preparation  of  the  present  volume  promptness  of  publication 
has  been  made  secondary  only  to  the  economic  utility  of  the  material 
presented.  The  papers  included  are  such  only  as  have  a  direct  eco- 
nomic bearing,  all  questions  of  purely  scientific  interest  being  excluded. 
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  detailed 
descriptions  of  occurrences  of  economic  interest,  noted  by  geologists 
of  the  Survey  in  the  course  of  their  field  work,  but  not  of  sufficient 
importance  to  necessitate  a  later  and  more  extended  description. 
The  papers  have  been  grouped  according  to  the  subjects  treated. 
At  the  end  of  each  section  is  given  a  list  of  previous  publications  on 
that  subject  by  this  Survey.  These  lists  will  be  serviceable  to  those 
who  wish  to  ascertain  what  has  been  accomplished  by  the  Survey  in 
the  investigation  of  any  particular  group  of  mineral  products.  They 
arc  generally  confined  to  Survey  publications,  though  a  few  titles  of 
important  papers  published  elsewhere  by  members  of  the  Survey  are 
included. 
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  Direc- 
tor.— Prior  to  1902  many  economic  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. — The  bulletins  of  the  Survey  comprise  a  series  of 
paper-covered  octavo  volumes,  each  containing  usually  a  single  report 
or  paper.  These  bulletins,  formerly  sold  at  nominal  prices,  are  now 
distributed  free  of  charge  to  those  interested  in  the  special  subject 
discussed  in  any  particular  bulletin.  This  form  of  publication  facili- 
tates promptness  of  issue  for  economic  results,  and  most  economic 
reports  are  therefore  published  as  bulletins.  Their  small  size,  how- 
ever, precludes  the  use  of  large  maps  or  plates,  and  reports  containing 
large  illustrations  are  therefore  issued  in  the  series  of  Professional 
Papers. 
3.  Professional  Papers. — This  series,  paper  covered,  but  quarto  in 
size,  is  intended  to  include  such  papers  as  contain  maps  or  other  illus- 
trations requiring  the  use  of  a  large  page.     The  publication  of  the 
