142 ELEMENTARY GEOLOGY 
Of the thousands of such caverns which pierce the 
limestone rock, only a very few have been opened to 
the eye of man, for entrance to most of them is diff- 
cult or even impossible. As the surface of the land 
melts down by denudation, these subterranean chambers 
may be partly or entirely opened to the air (Fig. 69). 
In some, like the Mammoth, fish are found in the 
underground river. — 
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Fie. 69. 
Diagram to illustrate exposure of caverns to the air by melting down of surface 
by denudation. This shows the same section as Fig. 67, after denudation has 
lowered the surface. In the meantime a lower cavern has been developed. 
The limestone is dissolved by water charged with 
carbonic acid gas; but as it enters the cave through 
the limestone roof, some of this gas escapes and the 
water is then forced to deposit a portion of its dis- 
solved carbonate of lime (Fig. 35). This may form 
pendent stalactites, or, droppmg to the floor, build 
stalagmites (Fig. 70). The downward growth of the 
stalactite may meet the rising stalagmite, forming a 
