Débris is the loose angular rubbish which collects at the base of cliffs, on hill-tops, and hill-slopes. Immense accumulations of it occur in lofty mountainous districts and in arctic regions. In Nova Zembla, for example, the solid rock of the country is almost concealed beneath a thick covering of débris. But the various kinds of débris will be more particularly described further on.
Soil.—An account of this can hardly be given without entering into the theory of its origin, and therefore we reserve its consideration for the present.
CHEMICALLY FORMED ROCKS.
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.
ORGANICALLY DERIVED ROCKS.