84 ELEMENTARY GEOLOGY 
solution. As the temperature decreases in contact 
with the cold air, some of this material must be pre- 
cipitated. So in the Yellowstone region, there are 
extensive beds of carbonate of lime formed near 
the hot springs (Fig. 33). When porous and spongy. 
in texture, this is called calcareous tufa or travertine. 
In the neighboring geysers (Figs. 220 and 221), silica 
is being brought up by 
the hot water; and as it 
is spread out in layers, 
it builds a rock which 
is called szlicious sinter 
(Fig. 34). Around hot 
springs other deposits 
are being made, and 
there is every reason to 
believe that at a depth 
Fic. 34. of hundreds or thousands 
Silicious deposit, Sapphire Pool, Yellow- of feet in the earth. these 
stone Park. 4 
hot spring waters are 
forming mineral veins (p. 380), perhaps of silver, gold, 
copper, or other metal. 
Deposit in Caves. A second important way in which 
rocks are chemically precipitated, is illustrated in cay- 
erns (p. 140), where pendent stalactites of carbonate of 
lime grow from the limestone roof at those places 
where water is slowly entering along some crevice 
