the valleys plunge into dark tunnels to reappear some miles away.  Ground water stands so far below the surface that it cannot be reached by wells, and the inhabitants depend on rain water stored for household uses.  The finest cavern of Europe, the Adelsberg Grotto, is in this region.  Karst, the name of a part of this country, is now used to designate any region or landscape thus sculptured by the chemical action of surface and ground water.  We must remember that Karst regions are rare, and striking as is the work of their subterranean streams, it is far less important than the work done by the sheets of underground water slowly seeping through all subsoils and porous rocks in other regions.

Even when gathered into definite channels, ground water does not have the erosive power of surface streams, since it carries with it little or no rock waste.  Regions whose underground drainage is so perfect that the development of surface streams has been retarded or prevented escape to a large extent the leveling action of surface running waters, and may therefore stand higher than the surrounding country.  The hill honeycombed by Luray Cavern, Virginia, has been attributed to this cause.

Cavern deposits.  Even in the zone of solution water may under certain circumstances deposit as well as erode.  As it trickles from the roof of caverns, the lime carbonate which it has taken into solution from the layers of limestone above is deposited by evaporation in the air in icicle-like pendants called stalactites.  As the drops splash on the floor there are built up in the same way thicker masses called stalagmites, which may grow to join the stalactites above, forming pillars.  A stalagmitic crust often seals with rock the earth which accumulates in caverns, together with whatever relics of cave dwellers, either animals or men, it may contain.

Can you explain why slender stalactites formed by the drip of single drops are often hollow pipes?

The zone of cementation.  With increasing depth subterranean water becomes more and more sluggish in its movements and more and more highly charged with minerals dissolved from the rocks above.  At such depths it deposits these minerals in the pores of rocks, cementing their grains together, and in crevices and fissures, forming mineral veins.  Thus below the zone of solution where the work of water is to dissolve, lies the zone of cementation where its work is chemical deposit.  A part of the invisible load of waste is thus transferred from rocks near the surface to those at greater depths.

As the land surface is gradually lowered by weathering and the work of rain and streams, rocks which have lain deep within the zone of cementation are brought within the zone of solution.  Thus there are exposed to view limestones, whose cracks were filled with calcite (crystallized carbonate of lime), with quartz or other minerals, and sandstones whose grains were well cemented many feet below the surface.