182 
SILICIOUS SPRINGS. 
ingredients with which springs are impregnated is^ 
small m comparison to hme, yet many of thera, 
especially thermal springs, contain more or less si- 
licious matter in solution. Indeed, to dissolve sili- 
ca, it is necessary that water should be raised to a 
high temperature. 
The hot springs of Iceland, called Geysers, afford 
the most remarkable example of the deposition of 
silex. The circular reservoirs into which the water 
falls are filled in the middle with a variety of opal, 
and round the edges with a siliciaus deposite called 
sinter. In some places a vesicular rock is formed, 
containing portions of vegetables more or less 
completely silicified ; and among other deposites is 
that of clay and silica, called tnpoli. The silex is 
deposited because the water, when cooled by ex- 
posure to the air, is unable to hold as much silica 
in solution as when it first issues from the earth, at 
a temperature of near 200^ of Fahrenheit. 
Dr. Webster describes the hot springs of St. Mi- 
chael^ in the Azores, as rising through volcanic 
rocks with a temperature varying from 93^ to 
Fahrenheit, and depositing vast quantities of sili- 
cious matter, which envelops the grass, leaves, 
and other vegetable substances that fall within their 
reach. The&e they render more or less fossil. The 
vegetables may be observed in every state of petri- 
faction* He found branches of the ferns, which 
now flourish in the island, completely petrified, pre- 
serving the same appearance as when vegetating, i 
excepting the colour, which is now ash-gray. Frag- I 
ments of wood occur more or less changed ; and 
one entire bed, from three to five feet in depth, is 
composed of the reeds,, so common in the island, i 
completely mineralized, the centre of each joint j 
being filled with delicate crystals of sulphur. H 
The siUcious deposites are both abundant and va- 
rious ; the most abundant occur in layers from a j| 
quarter to half an inch in thickness^ accumulated t# \\ 
