A&£mbe£W!°'}     Vegetation  of  Yellowstone  Hot  Springs.  631 
the  algae  present  a  different  development,  forming  leathery  sheets  of 
tough  gelatinous  material,  with  coralloid  and  vase-shaped  forms  ris- 
ing to  the  surface,  and  often  filling  up  a  large  part  of  the  pool. 
Sheets  of  brown  or  green,  kelpy  or  leathery,  also  line  the  basins  of 
warm  springs  whose  temperature  does  not  exceed  1400  F.,  but  in 
springs  having  a  higher  temperature  the  only  vegetation  present 
forms  a  velvety,  golden-yellow  fuzz  upon  the  bottom  and  sides  of  the 
bowl.  This  growth  is  rarely  noticed  in  springs  where  the  water 
exceeds  1600  F.,  except  at  the  edge  of  the  pool.  If  the  basin  is 
funnel-shaped,  with  flaring  or  saucer-shaped  expansion,  algae- grow 
in  the  cooler  and  shallower  water  of  the  margin,  forming  concentric 
rings  of  yellow,  old  gold  and  orange,  shading  into  salmon-red  and 
crimson,  and  this  to  brown  at  the  border  of  the  spring.  Around 
such  springs  the  growth  at  the  margin  often  forms  a  raised  rim  of 
spongy,  stiff  jelly,  sometimes  almost  rubber-like  in  consistency,  and 
red  or  brown  in  color.  Evaporation  of  the  water  drawn  up  to  the 
top  of  such  rims  leaves  a  thin  film  of  silica,  which  thickens  to  a  crust 
and  so  aids  in  the  production  of  a  permanent  sinter  rim." 
Near  some  springs,  for  example  near  the  Emerald  Pool,  algal 
channels  are  formed  and  the  waterway  is  floored  with  a  sheet  of 
olive  or  emerald  green,  kelpy  jelly.  Where  there  is  a  moderate 
current,  this  lining  is  nearly  smooth,  resembling  a  sheet  of  wet 
leather,  but  in  quieter  waters  this  soft  carpet  is  dotted  with  little 
warty  excrescences,  and  little  pillars  produced  by  the  upward  growth 
of  the  algae ;  the  pillars  sometimes  terminate  by  balloon-like  caps 
or  globes  containing  bubbles  of  gas.  When,  by  their  upward 
growth,  these  pillars  reach  the  surface  of  the  pool,  they  increase 
rapidly  in  diameter,  and  form  flat,  cap-shaped  formations  which 
sometimes  merge  into  table-like  expansions  of  quite  peculiar  form# 
The  continued  growth  of  new  pillars  dams  up  the  outlet,  and  the 
water  collecting  forms  shallow  lagoons  or  pools  of  varying  degrees 
of  temperature.  As  the  temperature  changes,  the  nature  of  the 
growth  changes,  the  bright-colored  algous  jelly  forming  the  outer 
covering  of  the  pillars  changes  to  light  salmon-pink,  and  the  sub- 
stance itself  becomes  noticeably  silicious,  or  forms  a  filmy  web  upon 
the  silicious  centre. 
It  has  been  for  some  time  known  that  the  hot  springs  of  the 
world  support  various  growths  of  microscopic  plants.  Agardh  and 
Corda  recognized  and  described  such  in  the  hot  springs  of  Carlsbad, 
