Geological Causes of the Scenery of Yelloiostoiie Park. 165 
contain principjilly silica, calcium carbonate, sodium borate, 
sodium arsenate, and sodium chloride. One-third to one-tifth 
of all their content ho\A-ever is silica. It amounts to about 
six- tenths grammes to a thousand of water. It is derived 
from the decomposition of the acid rhyolite. This shows how 
dependent the peculiaritif-s of the park are upon the material 
out of which it is constructed. Foi- if the underlying rock 
were limestone the material, though soluble, would not form 
geysei'S. If it were sandstone it would be practicall3^ insolu- 
ble because of absence of alkalies to aid in the decomposition. 
The deposit which is called siliceous sinter consists mainly of 
silica with varying amounts of water. If it is glassy it is 
called rhyolite. If it contains less than six per cent of water 
it is called pealite ; if from six to thirteen per cent, ge3'serite ; if 
seventj^-tive per cent, viandite, an unstable leather}' substance 
which crumbles when (\vy. The geyserite is deposited over 
an enormous space. In thirty years not more than one incli is 
formed. The conduit of Old Faithful is at least seventy feet 
deep. This would indicate that for twenty-five thousand years 
the deposit has been taking place. 
The cause of the deposition has long been a puzzle. It was 
thought that the cooling of the water would furnish an expla- 
nation. But demijohns of water standing two years gave no 
precipitate. However when the water was frozen precipita- 
tion did take place. It is now known that the deposits are 
due to three causes, a chemical, a ph3'sical, and a biological. 
The chemical is shown by the fact that the amounts of alka- 
line chlorides and carbonates present influence deposition ; 
the physical, by the fact that evaporation causes sedimenta- 
tion; and the biological by the fact that plants of so low an 
order of development that they consist of hardlj' anything 
more than a mass of gelatine, growing in the water, form in 
their bodies siliceous spicules and become incrusted with silica. 
This variet)' of causes results in a variety of forms, and 
the deposits resemble at times cauliflower, cabbage, mosses, 
leaves, sponges, corals, stalactites, spicules and spheres. 
Sometimes the material piles up in regular forms as the Bee 
Hive geyser, or in bizarre forms like the Grotto, or in great 
masses like the White Pyramid, or in rounded craters like Old 
Faithful, or in flat forms without elevation like the Excelsior. 
