AD^cJemberP Sfl  }     Vegetation  of  Yellowstone  Hot  Springs.  627 
effects.  No  one  now  goes  to  the  Park  on  account  of  its  mineral 
waters.  It  would,  therefore,  be  premature  to  assume  that  there  is 
no  medicinal  virtue  in  them.  Two  great  drawbacks  are  to  be 
encountered,  and  these  alone  are  sufficient  to  explain  why  the  Yel- 
lowstone will  probably  never  become  a  resort  for  invalids.  Inacces- 
sibility, length  and  severity  of  the  winters  are  sufficient  obstacles 
to  the  National  Park  ever  becoming  such  a  resort.  The  open 
summer  season  lasts  only  about  three  months. 
The  hot  springs  and  geysers,  on  the  other  hand,  are  interesting 
to  the  geologist,  because  of  the  remarkable  phenomena  connected 
with  their  origin  and  activity ;  to  the  botanist  they  are  fascinating, 
because  of  the  low  forms  of  vegetal  life  found  existing  in  them 
even  at  high  temperatures. 
As  before  stated,  the  waters  which  run  from  the  hot  springs  and 
geysers  of  the  Yellowstone  may  be  comprehended  under  two  heads — 
those  which  deposit  silica,  as  sinter,  and  those  which  form  calcium 
carbonate,  as  travertine.  The  last-mentioned  substance  is  only 
found  in  the  Mammoth  Hot  Spring  Basin ;  the  latter  makes  up  the 
characteristic  formations  of  the  Norris,  Lower  and  Upper  Geyser 
Basins.  The  question  naturally  arises,  how  are  the  beautiful  terraces 
which  surround  many  of  the  hot  spring  centres  formed  ?  Are  they 
not  simply  built  up  by  the  deposition  of  new  material  from  the  over- 
flow water,  as  it  evaporates  and  cools  at  the  surface  ?  At  first  sight, 
it  would  seem  that  the  craters  and  bowls  of  the  geysers  and  hot 
springs  were  foimed  in  this  way,  because  we  know  that  boiling 
water,  under  pressure,  will  dissolve  and  hold  in  solution  much  more 
inorganic  material  than  ordinary  river  or  spring  water  at  the  normal 
temperature,  and  that  in  many  instances,  when  the  pressure  is 
relieved  and  the  temperature  lowered,  the  water  will  precipitate  its 
mineral  contents. 
In  the  case  of  the  richly  carbonated  waters  of  the  Mammoth  Hot 
Springs,  calcium  carbonate  is  deposited  by  the  relief  of  pressure,  by 
the  escape  of  the  carbon  dioxide  and  by  the  evaporation  of  the 
water ;  but  this  physical  process  is  not  the  sole  cause  of  the  varied 
and  beautiful  terraces,  which  will  presently  be  described.  At  the 
Norris  Geyser  Basin,  relief  of  pressure  and  cooling  will  cause  a 
separation  of  silica  from  the  hot  waters,  but  the  waters  of  the  other 
geyser  basins  contain  very  much  less  silica,  and,  as  far  as  has  been 
observed  by  geologists,  neither  relief  of  pressure  nor  cooling  will 
