146 
Dry formation is generally dazzling white, 
as pure and clear as any snow field of the 
Alps or Norway. Dry formation reflects 
the light from innumerable crystals and the 
rays all seem to focus upon the individual 
in the proximity. This is why the forma- 
tion isso hotand blistering. This is why the 
kodak must be ‘stopped down’ to its 
smallest diaphragm if one desires to photo- 
graph the geysers, as many a tourist has 
learned to his sorrow when he viewed the 
dark brown, over-exposed film of some 
geyser in action, and then remembered the 
long wait in the glittering sunshine, the 
madrush for favorable points of view and 
the supreme satisfaction that he felt when 
the exposure was indexed and described : 
‘Old Faithful from the west, sunshine 
upon the vapor clouds, fir forest in the 
background.’ 
Wet formation is usually colored. Wher- 
ever a stream of water flows from a hot 
spring the course is marked by streaks of 
green and yellow along its margin. Fringes 
and rims of green and gamboge border the 
boiling pools. Upon the sides of the pulpit 
basins (see Fig. 1) are flutings of red and 
orange, of green and brown. The sides and 
bottoms of the streams are lined with olive 
colored felts of a velvet-like consistency, 
and there may be green or brown stringy 
filaments waving to and fro in the hot cur- 
rents of water. The sides of many of the 
pools are covered with green, leathery mem- 
branes, or smeared with a coating of struc- 
tureless jelly. Even the hottest pools, those 
which are boiling violently in certain parts, 
are likely to have some of the colored 
fibrous or gelatinous deposits at their edges. 
The temperature of the water in these 
streams and basins is frequently above 30° 
C., and water boils in Yellowstone Park in 
the neighborhood of 92° C. 
Almost all of this brilliant coloring is 
associated with growths of low forms of 
plant life. Some of these organisms it will 
SCIENCE. 
[N. 8S. Vou. VI. No. 135. 
be our purpose to describe, together with a 
discussion of certain activities manifested 
by them in connection with the structure 
and development of the formation. It is 
not difficult to see that the growth is vari- 
ous in character in different parts of the 
hot springs. One finds the most luxuriant 
masses in such portions of the streams as 
have cooled down to a relatively low tem- 
perature, that is where the water, instead 
of being scalding hot, may be simply un- 
bearably warm. 
Here, at a temperature of from 40°-50° 
C., flourish a variety of forms with the great- 
est display of color. The prevailing tints 
are green, but much of the growth is 
brown, red or frequently orange. The forms 
which the masses of vegetation assume de- 
pend upon the condition of the environ- 
ment. If there is a rapid stream the bottom 
will be covered by a smooth, slippery, 
leathery felt, and perhaps occasional stringy 
masses attached at points along the sides 
and bottom will float out with the current. 
If water lies in a quiet pool one may find a 
thick growth of this felt-like character on 
the bottom and there will be numerous 
tufts perched upon projecting knobs of 
rocks. These tufts are made especially 
prominent by the bubbles of gas entangled 
in the network of filaments. The tufts are 
particularly interesting because they give 
rise to some of the most peculiar sculpturing 
of the shallow pools. 
In warm water, at a temperature of from 
55°-65° ©. there is a preponderance of 
green growth, much of it a vivid emerald 
green. The color becomes less pronounced 
in warmer portions of the stream, fading to 
a yellowish brown, and finally becoming 
light yellow when the water approaches 80° 
C. in temperature. In the hottest water one 
finds only white filaments, which grow as 
long silky strings in the running streams or 
form a delicate cobweb upon the bottoms of 
quiet pools. This is the growth that marks 
