_Talc_ is a silvery white, grayish, pale or dark-green coloured mineral, with a pearly lustre. It splits readily into thin flakes, which are flexible, but not elastic, and may be readily scratched with the nail. It is unctuous and greasy to the touch. It occurs in beds (_talc-slate_), and is often met with in districts occupied by metamorphic crystalline rocks. _Serpentine_ is generally of a green colour, but brown, red, and variously mottled varieties occur. It has a dull lustre, and is soft, and easily cut; it is tough, however, and takes on a good polish. It forms rock-masses in some places. The finer varieties are called _noble serpentine_. _Chlorite_ is another soft, easily scratched mineral, generally of a dark-green colour. It has a pearly lustre. Sometimes it occurs in beds (_chlorite-slate_), and is often found coating the walls of fissures in certain rocks. It has a somewhat greasy feel. The three last-mentioned minerals--talc, serpentine, and chlorite--are all silicates of magnesia. _Zeolites_ is a term which comprises a number of minerals of varying chemical composition, all of which tend to form a jelly when treated with acids. When heated by the blow-pipe they bubble up, owing to the escape of water; hence their name _zeolites_, from _zeo_, I boil, and _lithos_, a stone. The zeolites occur very commonly in cavities in igneous rocks, and also in mineral veins. Having now mentioned the chief rock-forming minerals, we proceed to a brief description of some of the more typical representatives of the five great classes of rocks referred to at page 8. PETROLOGY.[C] [C] _Petros_, a rock, and _logos_, a discourse. Some geologists restrict this term to the study of the _structure_ and _arrangement of rock-masses_, and apply the term _lithology_ (_lithos_, a stone, and _logos_, a discourse) to the study of the _mineralogical composition of rocks_. MECHANICALLY FORMED ROCKS. 17. (_A._) SEDIMENTARY CLASS.--Three of the most commonly occurring rocks of this class have already been described, but a few details are added here.