CHAPTER III. 



THE CHIEF MINEEALS WHICH FORM THE EARTH. 



IT has already been seen that minerals maybe now in process of formation 

 in the sun, and the fixed stars ; that they form meteorites, and compose 

 the earth's surface. It will be a sufficient definition of a mineral to 

 say that it is the natural condition in which the substances that form 

 rocks exist in' the earth. Chemists have classified the constituents of 

 the earth into elementary bodies which no analysis has yet been able 

 to further subdivide. Some of these elements, more or less pure, 

 occasionally constitute minerals, such as sulphur, carbon, copper, 

 silver, gold. But more frequently minerals consist of several ele- 

 mentary bodies chemically combined, and then each compound thus 

 made up, is met with usually in a series of geometrical crystalline 

 shapes, and each has a distinctive hardness, colour, mode of cleavage 

 or crystalline splitting, and other peculiarities, by which it may be 

 more or less easily distinguished from other minerals. Thus the 

 mineral galena, a well-known bluish-grey metallic-looking ore of lead, 

 consists of a chemical compound of lead and sulphur, which crystallises 

 in some modification of the cube or octahedron, and readily cleaves 

 parallel to the faces of the cube. 



The abundant chemical elements in the earth are remarkably few, 

 and may be enumerated as oxygen, silicon, aluminium, calcium, sodium, 

 potassium, iron, manganese, magnesium, lithium, chromium, carbon, 

 barium, sulphur, chlorine, nitrogen, fluorine, and hydrogen, which is 

 usually present in combination with oxygen forming water. These 

 elements, variously combined with each other, constitute the minerals 

 which compose rocks, and though occasionally minute quantities of 

 other elements occur in rocks, yet not more than half of those named 

 will usually be found. The mineral substances constituting aqueous 

 or water-formed rocks, are massive and rarely crystallised, though 

 various crystallised minerals occur in them ; and as the minerals of 

 water-formed rocks are usually different in character and appearance 

 from the same substances when found in igneous rocks, it may bo 

 convenient to enumerate them separately in tabular form. They 

 include : 



