14. Nearly all the minerals we know of contain oxygen as a necessary ingredient, there being only a very few minerals in which that gas does not occur in chemical union with other elements. Three of these minerals, _sulphur_, the _diamond_, and _graphite_, consist of simple substances, and are of great commercial importance, but none of them is of so frequent occurrence, as a rock constituent, as the minerals presently to be described. _Sulphur_ occurs sometimes in thin beds, but more frequently in small nests and nodules, &c. in other rocks, or in joints, and fissures, and veins. It is frequently found in volcanic districts. The _diamond_, which consists of pure _carbon_, is generally met with in alluvial deposits, but sometimes, also, in a curious flexible sandstone, called _itacolumite_. _Graphite_ is another form of carbon. It occurs both in a crystalline and amorphous form, the latter, or non-crystalline kind, being extensively used for lead-pencils. _Rock-salt_ is a _chloride of sodium_, and appears sometimes in masses of a hundred feet and more in thickness. Another mineral which contains no oxygen is the well-known _fluor-spar_. It occurs chiefly in veins, and is often associated with ores. With these, and a few other exceptions, all the minerals hitherto discovered contain oxygen as an essential element; and so large is the proportion of this gas which enters into union with other elements to constitute the various minerals of which the rocks are composed, that it forms at least one-half of all the ponderable matter near the earth's surface. When the student learns that there are probably no fewer than six or seven hundred different minerals, he will understand how impossible it is to do more in a short geological treatise than point out a few of the most commonly occurring ones. And, indeed, a knowledge of the chief rock-forming minerals, which are few in number, is all that is absolutely requisite for geological purposes. Some of these we accordingly proceed to name.[B] [B] It is needless to describe the minerals minutely here. The student can only learn to distinguish the different species by carefully examining actual specimens. 15. _Quartz._--This mineral has already been partially described. It is the most abundant of all the rock-forming minerals, and occurs in three forms: (1) _crystallised quartz_ or _rock crystal_; (2) _chalcedony_, both of which are composed of silica--that is, silicon and oxygen; and (3) _hydrated quartz_--that is, silica with the addition of water. _Hematite._--This is an oxide of iron. It occurs in mammillary rounded masses, with a fibrous structure, and a dull metallic lustre. _Magnetite_ or magnetic iron ore, _specular iron_, and _limonite_ are also oxides of iron. _Hematite_ shews a red streak when scratched with a knife, which distinguishes it from magnetite.