Agate— Physical Properties and Origin 7 
for many years were accepted by most investigators. 
The theory is stated by Bauer in a complete form as 
follows: "To explain this phenomenon [the forma- 
tion of agates] the existence is assumed of hot, inter- 
mittent springs, such as are now seen to perfection 
in the geysers of Iceland and in the United States 
National Park on the Yellowstone River. The essen- 
tial condition is that the hot or warm water rising 
up from the depths shall saturate the rocks, and that 
it shall sink again, leaving the rocks dry for a period. 
The hot water dissolves out the silica and other con- 
stituents of the rock and the solution fills up the 
amygdaloidal cavities. When the waters sink, these 
cavities are emptied, only a film of water covering 
their walls being left behind. On the evaporation 
of this film, which readily takes place at such a high 
temperature, a thin layer of silica is deposited on the 
walls of the cavity. When the hot spring again rises, 
the same thing takes place and a second layer of silica 
is deposited, and so on, until the cavity is more or less 
completely filled up. Each time, the fluid passes into 
and out of the cavity by the canals, which are for 
this reason often referred to as tubes of entry or 
escape. The fluid, also, perhaps, passes to a certain 
extent through the porous agate itself. Crevices, fis- 
sures, and other cavities in the rock are, of course, 
filled with agate in the same way." 
It is only recently that a theory for the formation 
of agates has been proposed which gives a totally 
different explanation from the above and which is in 
many respects more satisfactory than the earlier views. 
According to the later theory, the rock cavity in which 
the agate is formed, first becomes more or less filled 
with silica in a colloidal (jelly-like) condition. In such 
a colloid a banded structure can be produced by proc- 
esses which can be illustrated with ordinary gelatine. 
[in] 
