20. _Stalactites_ and _stalagmites_ are carbonates of lime. They vary
in colour, being white, or yellow, or brown. Stalactites are usually
found adhering to the roofs of limestone caverns, &c., or depending from
limestone rocks; stalagmites, on the other hand, commonly occur on the
floors of limestone caverns, where they often attain a thickness of many
feet.
_Siliceous sinter_ is silica with the addition of water--in other words,
a hydrated quartz. It is not a very abundant rock, and is found chiefly
in volcanic countries.
_Rock-salt_ has already been described. It occurs either as thin beds,
or in the form of thick cake-like masses, often reaching ninety or one
hundred feet in thickness. It is rudely crystalline in texture, and is
usually discoloured brown and red with various impurities.