400 The American Naturalist. [May, 
fuzz of green, through which the red tint beneath shows and 
produces an olive tone. 
The algae tinting the hotter laugs, with temperatures of 
140° to 160^ Fahr., are bright yellow, and form a loose, vel- 
vety nap on the soft, siliceous sediment. 
Where the overflow from a spring is constant in volume the 
channels are rapidly filled, choked and dammed back by 
masses of red and green algous jelly from one-half to five 
inches thick. This form of growth and the process of sinter 
formation has been already described elsewhere. ' The channels 
carrying off the periodic discharge of the geysers are also 
brilliantly tinted by algae, but modified by the deposit of 
silica. The channels of Old Faithful are a brilliant gamboge 
yellow near the geyser, merging into orange, which changes 
abruptly into brown, while farther away the growth is cedar 
red. 
In these cases the plants form a thin, slippery coating upon 
the siliceous sinter, and is much encrusted by silica. Where 
from any cause the alga growing in these channels are de- 
prived of their supply of water, the siliceous jelly enveloping 
the growth is rapidly dried, and becomes hard, white and 
opaque, effectually concealing the algae. Where channels are 
lined with a membranous growth, this shrivels up into curious 
convoluted forms, or into papyrus-like rolls. In fact, what- 
ever the nature of the algas present in the siliceous waters, all 
appearance of vegetable life is soon lost on drying, owing to 
the hardening of the silica. In calcareous waters the change 
is none the less complete, and the green or red growth rapidly 
bleaches out and becomes all but invisible to the casual ob- 
server in the deposit. The filaments may, however, be freed 
from the lime by the aid of acid. 
' Amer. Journ. Sci. May, 1889. 
