SEDIMENTARY AND METAMORPHIC ROCKS 85 
(Fig. 35). This water carries carbonic acid gas; and, 
so charged, is able to dissolve some of the carbonate 
of lime of the rock through which it passes. When 
the water enters the cave, a portion of the carbonic 
acid gas escapes. The solvent power of the water is 
Fic. 35. 
Stalactites ina cave. (Copyright, 1889, by S. R. Stoddard, Glens Falls, N. Y.) 
then decreased, so that, of necessity, some of the lime 
is deposited in the form of a stalactite. In somewhat 
the same way, deposits of bog iron ore are formed at 
the outlet of iron-bearing springs, where the ferrugi- 
nous waters come in contact with the air. 
