_Augite_ or _Pyroxene_ has, till recently, been considered as a variety of hornblende. Its specific gravity is slightly different; its composition is the same, and in general appearance it is not easily distinguished from hornblende. It has, however, been made a distinct species, because its crystalline form is different. _Hypersthene_ is composed of silex, magnesia and oxide of iron. Its specific gravity is 3.38. It closely resembles hornblende. The lustre of its cleavage faces is metallic pearly. Its color is grayish or greenish black. _Talc_ is composed of silex and magnesia. Its specific gravity is 2.7. It resembles mica in its general appearance and in its lamellar structure, but it is easily distinguished from it by its plates being not elastic, and by its soapy feel. Its color is generally some shade of green. _Soapstone_ is an impure variety of talc, of a light gray color, earthy texture, and is unctuous to the touch. _Chlorite_, another impure variety, is a dark green rock, massive, easily cut with a knife, and unctuous to the touch. _Serpentine_ is composed of silex and magnesia. Its specific gravity is 2.55. It is generally massive, unctuous to the touch, and of a green color. It is often variegated with spots of green of different shades. With a mixture of carbonate of lime it forms the _verd antique marble_. _Carbonate of Lime_, or common limestone, is composed of carbonic acid and lime. Its specific gravity is 2.65. It presents a great variety of forms. In a crystalline state it is generally transparent, and when so, possesses the property of double refraction. It may be distinguished from every other common species by its rapid effervescence with acids. It readily cleaves parallel to all the faces of the primary form, which is a rhombohedron. _Sulphate of Lime_, or Gypsum, is composed of sulphuric acid and lime. Its specific gravity is 2.32. When crystalline, it has a pearly lustre, is transparent, and goes under the name of _Selenite_. _Common Gypsum_ resembles the other earthy limestones, but it is softer, and may be readily distinguished by its not effervescing with acids. To the minerals now enumerated may be added the following, which are of frequent occurrence, but not in great quantities; namely, carbonate of magnesia, oxide of iron, iron pyrites, rock-salt, coal, bitumen, schorl and garnet. These simple minerals, either in separate masses or mingled more or less intimately together, compose almost wholly the earth's crust. SECTION III.--THE MINERAL MASSES WHICH FORM THE CRUST OF THE EARTH. That portion of the structure of the earth which is accessible to man is called the _crust of the earth_. The mineral masses which compose it, whether in a solid state, like granite and limestone, or in a yielding state, like beds of sand and clay, are called _rocks_. The _unstratified rocks_ are Granite, Hypersthene rock, Limestone and Serpentine, and the Trappean and Volcanic rocks.