140 ELEMENTARY GEOLOGY 
either in the form of a spring or by gently oozing from 
the soil or rock. We may see the latter on the face 
of any cliff after the melting of the winter snow; the 
former mode of return is familiar to all. The under- 
ground water changes the minerals (p. 117), and at the 
end of its journey always brings to the surface a cer- 
tain small amount of rock material in solution. This 
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Fia. 67. 
Diagram to illustrate formation of limestone caves. Water enters through sink- 
holes and after passing on an underground journey in the caverns, reappears 
at the surface as a spring (s). 
robs the soil of some of its constituents, and adds them 
to the river, which in turn carries them to the sea. 
Limestone Caves. — Water sinking into the ground 
will follow the easiest channel open to it; and as there 
are numerous natural breaks in the rocks, these com- 
monly serve as passageways. If the rock which the 
water meets is a soluble one, like limestone, these 
natural channels may be slowly enlarged (Fig. 67). By 
