_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. 21. _Limestone_ consists of carbonate of lime, but usually contains some impurities. The varieties of this rock are numerous; some of them are as follows: _Chalk_; _oolite_, a rock built up of little spheroidal concretions, whence its name, _egg_ or _roe stone_ (the coarser oolites are called _pisolite_, or _pea-stone_); _lacustrine limestone_, &c. When much silica is diffused through the rock, we have a _siliceous limestone_; the presence of clay and of carbonaceous matter gives us _argillaceous_ and _carbonaceous limestones_. _Cornstone_ is a limestone containing a large quantity of arenaceous matter or sand. Many limestones are distinguished by the different kinds of organic remains which they yield. Thus, we have _muschelkalk_ or _shell-limestone_, _nummulitic_, _crinoidal_, &c. limestone. The crystalline limestones, such as _statuary marble_, are metamorphosed limestones. Not a few limestones are chemically formed rocks, and many, also, are partly of chemical and partly of organic origin, so that no hard and fast line can be drawn between these two classes of rock. _Dolomite_, or _magnesian limestone_.--This is a compound of carbonate of lime and carbonate of magnesia. Its colour is usually yellow, or yellowish brown, but gray and black varieties are sometimes met with. It is generally fine-grained, with a crystalline texture, and pearly lustre. It effervesces less freely with acids than pure limestone. In many cases dolomite is merely a metamorphosed limestone.