22 MINERALS WHICH FORM IGNEOUS ROCKS. 



Among the other minerals which form beds of rock are coal, the 

 various oxides of iron, and carbonate of iron ; which we shall describe 

 in another place. And among minerals found in the strata are 

 alum, barytes, &c. For technical descriptions of these species refer- 

 ence may be made to a treatise on mineralogy such as Dana's, or 

 Rutley's, or Phillips' Mineralogy by Brooke and Miller. And in any 

 case, the best knowledge of them will be obtained by examining the 

 specimens in some public collection, such as the British Museum or 

 Museum of Practical Geology, or the Museums of our Universities. 



The abundant minerals which form igneous rocks are rather more 

 numerous, but comprise chiefly the felspars, augites, hornblendes, 

 micas, talcs, quartz, olivine, garnets, zeolites, and a few others. The 

 more interesting general facts relating to the species of these groups 

 which bear upon rock structure may be thrown into a tabular form 

 for easy reference. Of quartz nothing further need now be said. 



THE MINERALS FORMING IGNEOUS ROCKS. 

 The Family of Felspars, 



Felspars are the most abundant minerals in igneous rocks. They 

 can be just scratched with a knife, being softer than quartz, harder 

 than apatite, and much harder than carbonate of lime. The colour 

 is often milky- white, sometimes bright red owing to the presence of 

 oxide of iron, and occasionally grey or black, or even green. All 

 felspars consist chemically of silicates of alumina combined with 

 some other silicate, which is usually silicate of potash, or soda, or 

 lime, or some combination of lime and soda ; and according to 

 variations in chemical composition, the different varieties or species 

 of felspar are identified and named. With these chemical differences 

 are associated differences of crystalline form. When a typical 

 felspar contains potash, it crystallises in prisms in what is called 

 the oblique or monoclinic system, and is recognised by fracturing at 

 right angles to the side of the prism ; but when the crystal contains 

 soda or lime it crystallises in the doubly oblique or triclinic system, 

 and the cleavage is then at an oblique angle. For most purposes it is 

 sufficient to identify these two groups known as orthoclase and plagio- 

 clase, which can almost always be recognised even in microscopic 

 examples by the different ways in which they affect light, when 

 examined in thin slices, under the microscope, with the aid of the 

 polariscope. There are, however, at least six species of felspar pro- 

 perly so called, which are named orthoclase, oligoclase, albite, labra- 

 dorite, anorthite, and andesine ; the nearly allied minerals which may 

 replace the felspars in igneous rocks are nepheline, leucite, sodalite, 

 haiiyne, and noseau (Cotta's "Rocks Classified and Described;" 

 ZirkeFs "Lehrbuch der Petrographie.") 



