58 



LITHOLOGICAL GEOLOGY. 



Fig. 54. 



(rr 



I I i2 



Fig. 55. 



11% i i a 



Fig. 53, the appearance of the crystals in the rock (often quartz). Besides black, there 

 are also brown, green, red and white tourmalines. Constituents: silica, alumina, mag- 

 nesia, with fluorine and some boracic acid. 



Topaz, in rhombic prisms of 124° 19', remarkable for cleav- 

 ing with ease and brilliancy parallel to the base of the prism. 

 Colors, yellowish to white; also brown. Two of the forms of 

 its crystals are shown in figures 54, 55. 



Beryl, in six-sided prisms, usually pale green, but deep green 

 in the variety emerald. 



All the above anhydrous minerals are too hard to be 

 scratched with a file. The following contain water, and are 

 softer. 



2. Hydrous Silicates. — Besides the hydrous micas, there are the common species: 

 Agahnatolite , a compact mineral, soapy to the touch, often resembling a compact soap- 

 stone. Like serpentine and massive pyrophyllite, it is often cut into images in China. 

 Consists of silica, alumina, potash, and water. 



Pyrophyllite. — A mineral resembling talc in color, cleavage, and soapy feel, when 

 crystallized, and like some fine-grained soapstone when massive. Consists of silica, 

 alumina, and water. It differs from talc in containing alumina in place of magnesia. 



Glauconite or Green Earth, the material of the New Jersey marl, or Green sand of the 

 Cretaceous and other rocks. It is a soft, dark or light green silicate of alumina, iron, 

 and potash, with water. 



Clay is not ordinarily a simple mineral, but a mixture of powdered feldspar and 

 quartz. But the soft clay, soapy to the touch, found in some places, is the species hao- 

 linite, or the material of kaolin — the clay which is used in the manufacture of porcelain. 

 It is a result of the decomposition of some kind of feldspar containing potash or soda, 

 and consists of silica 41*5, alumina 34*4, and water 24*1 = 100. 



3. Carbonates, Sulphates, Phosphates, and Fluorids. — Among these min- 

 erals, there are a few species common enough to be here enumerated. 



Siderite or Carbonate of Iron, like calcite in cleavage, and white or grayish white, 

 but changing readily to a brown color on exposure, and finally to the hydrous oxyd of 

 iron called limonite. 



Magnesite, or Carbonate of Magnesia, white and 

 like calcite in cleavage, but often occurring massive 

 and looking like porcelain biscuit. 



For other related carbonates, reference must be 

 made to the Mineralogy. 



Barite, or Heavy Spar, is a sulphate of baryta. It 

 occurs in tabular crystals, some of the forms of which 

 are given in Fig. 56. It is remarkable for its high 

 specific gravity, whence the name, from the Greek 

 for -weight. It contains sulphuric acid 34 "33, baryta 

 65-67 = 100. 



Apatite is a phosphate of lime. It commonly oc- 

 curs in six-sided prisms, greenish in color, and look- 

 ing like beryl, from which it differs in being easily 

 scratched with a knife. Its crystals are sometimes 

 transparent and colorless, or bluish, and occasionally 

 brown. 

 Fluorite, or Fluor Spar, a fluorid of calcium. Its crystals are cubes, octahedrons, 

 and other related forms. All of them cleave easily in four directions, parallel to the 

 faces of the regular octahedron, the faces of cleavage making angles with one another 

 of 109° 28'. It is often granular-massive. It is easily scratched with a file. Its colors 

 are clear purple, yellow, blue, often white, and of other shades. When powdered and 

 thrown on a shovel heated nearly to redness, it phosphoresces brightly. Composition: 

 fluorine 48 - 7, calcium 51*3. 



Fig. 56. 



