iTES,]  INTRODUCTION.  13 
illetin.  This  form  of  publication  facilitates  promptness  of  issue  for 
jonomie  results,  and  most  economic  reports  are  therefore  published 
i  bulletins.  Their  small  size,  however,  precludes  the  use  of  large 
aps  or  plates,  and  reports  containing  large  illustrations  are  there- 
>re  issued  in  the  series  of  Professional  Papers. 
3.  Professional  Papers  of  the  United  States  Geological  Survey. — 
his  series,  paper  covered,  but  quarto  in  size,  is  intended  to  include 
lich  papers  as  contain  maps  or  other  illustrations  requiring  the  use  of 
large  page.  The  publication  of  the  series  was  commenced  in  1902, 
id  the  papers  are  distributed  in  the  same  manner  as  are  the  bulletins. 
4.  Monographs  of  the  United  States  Geological  Survey. — This  series 
msists  of  cloth-bound  quarto  volumes,  and  is  designed  to  include 
liaustive  treatises  on  economic  or  other  geologic  subjects.     Volumes 
-  this  series  are  sold  at  cost  of  publication. 
5.  Geologic  folios  of  the  United  States  Geological  Survey. — Under 
te  plan  adopted  for  the  preparation  of  a  geologic  map  of  the  United 
bates  the  entire  area  is  divided  into  small  quadrangles,  bounded  by 
rtain  meridians  and  parallels,  and  these  quadrangles,  which  number 
veral  thousand,  are  separately  surveyed  and  mapped.  The  unit  of 
irvey  is  also  the  unit  of  publication,  and  the  maps  and  descriptions 
-  each  quadrangle  are  issued  in  the  form  of  a  folio.  When  all  the 
)lios  are  completed  they  will  constitute  a  Geologic  Atlas  of  the 
nited  States. 
A  folio  is  designated  by  the  name  of  the  principal  town  or  of  a 
•ominent  natural  feature  within  the  quadrangle.  It  contains  top- 
graphic,  geologic,  economic,  and  structural  maps  of  the  quadra n- 
e,  and  occasionally  other  illustrations,  together  with  a  general 
ascription. 
Under  the  law,  copies  of  each  folio  are  sent  to  certain  public  li bra- 
es and  educational  institutions.  The  remainder  are  sold  at  25  cents 
,ch,  except  such  as  contain  an  unusual  amount  of  matter,  which  are 
"iced  accordingly. 
Circulars  containing  lists  of  these  folios,  showing  the  locations  of 
Le  quadrangular  areas  they  describe,  their  prices,  etc.,  are  issued 
om  time  to  time,  and  may  be  obtained  on  application  to  the  Director 
I  the  United  States  Geological  Survey.  The  following  list  shows 
e  folios  issued  to  date  and  the  economic  products  discussed  in  the 
xt  of  each,  the  products  of  greatest  importance  being  printed  in 
ilic. 
