628  Vegetation  of  Yellowstone  Hot  Springs.  {^l^x^' 
produce  a  separation  of  the  silica.  Water  collected  from  the  springs 
and  geysers  of  the  Upper  and  Lower  Geyser  Basins  was  perfectly 
transparent,  and  remained  clear  and  without  sediment  after  standing 
for  several  years.  Experiments  showed  that  the  silica  in  these 
waters  remained  dissolved,  even  when  the  water  was  cooled  down  to 
the  freezing  point,  and  it  was  only  after  the  crystallization  of  the 
water  by  freezing  that  the  silica  was  separated  and  settled  down  as 
an  insoluble  flocculent  precipitate  upon  melting  the  ice. 
How,  then,  are  we  to  account  for  the  production  of  the  exquisite 
terraces,  mounds,  pools  and  geyser  cones?  It  has  been  proved,  in 
addition  to  the  causes  operative  in  the  above  instance,  that  the  rapid 
deposition  of  the  sinter  and  travertine  from  both  classes  of  water  is 
due  to  the  action  of  vegetation  in  removing  the  carbon  dioxide  from 
carbonated  waters,  thus  depositing  calcium  carbonate,  and,  in  the 
case  of  the  silicious  waters,  depositing  by  the  activity  of  the  proto- 
plasm a  gelatinous  silica,  which,  upon  exposure,  finally  hardens. 
We  know,  from  numerous  observations,  that  plants  are  active  in 
rock  building  and  disintegration. 
The  plants  of  the  Carboniferous  Period,  by  their  death  and  con- 
solidation, formed  the  extensive  and  useful  coal  beds.  Sphagnum 
and  mosses  compacted  yield  peat,  and,  in  some  cases,  soft  coal.  Sili- 
cious diatoms  have  given  rise  to  extensive  diatomaceous  earths. 
In  several  of  the  higher  algae,  for  example,  Halimeda  opuntice,  the 
carbonate  of  lime  deposited  by  the  plant  forms  a  sieve-like  cover 
about  the  tips  of  the  algal  filaments,  and,  in  Acetabularia,  it  occurs 
as  a  tube  about  the  stalk  of  the  plant.  In  the  charge  the  lime  is 
separated  and  deposited  in  the  cells  and  cell  walls  of  the  back  alone, 
while  in  the  Corallines  it  is  found  only  within  the  cells.  Nor  is  our 
knowledge  of  the  activity  of  protoplasm  in  the  deposit  of  mineral 
substance  solely  confined  to  plants.  We  know  that  many  animals 
secrete  silex  and  carbonate  of  lime,  foraminifera,  coral  polyps  and 
molluscs  generally.  Before,  however,  we  can  understand  the  part 
which  vegetation  has  played  in  forming  the  travertine  and  sinter 
beds  of  the  Yellowstone  Park,  we  must  become  familiar  with  the 
general  appearance  and  character  of  the  deposits  themselves. 
First  in  importance  among  the  many  points  of  interest  accessible 
are  the  Hot  Spring  Terraces.  These  have  been  built  one  upon 
another,  until  the  present  active  portion  constitutes  a  hill  rising  300 
feet  above  the  site  of  the  Mammoth  Hot  Springs  Hotel.    The  for- 
