( 2 5 2 ) 



CHAPTER XVII. 

 THE HISTORY OF VOLCANIC KOCKS. 



Method of Study. Igneous rocks can only be studied with ad- 

 vantage in their natural occurrences on the surface of the country ; 

 and the student will always spend his time most profitably who gains 

 practical knowledge of the behaviour and variation of rocks in the 

 field by detailed study of some classical district. 



A useful preparation for such an investigation is a preliminary know- 

 ledge of the common minerals which combine to compose these rocks. 

 And this can hardly be attained better than by collecting the specimens 

 on the old lava streams of a volcanic district, such as the Auvergne, 

 Siebengebirge, or Eifel ; but in cases where such practical work is not 

 immediately convenient, specimens of rock-forming minerals (see p. 

 22) must be obtained from some trustworthy dealer. When their 

 crystalline forms and other physical properties have been sufficiently 

 examined, duplicate specimens of each should be sliced and mounted 

 for study under the microscope with polarised light, so as to form a 

 standard series, by which similar minerals may be recognised, when 

 they occur in combination in igneous rocks. Having the commoner 

 types of minerals thus prepared, and furnished with a microscope, to 

 which a polariscope is fitted, the student is qualified to begin practical 

 work. He must prepare, or have slices prepared from typical volcanic 

 rocks, and will soon be able to identify the constituent minerals in 

 them by comparison with his types, so that the rock slices will be- 

 come a series of standards. He will then be in a position to add to 

 the store of knowledge by determining the nature of the rocks found 

 in districts which may come under examination. 



The Characters of Minerals in Polarised Light. The common 

 rock-forming minerals can often be identified under the -ifiicroscope 

 by their crystalline forms and method of arrangement ; but under 

 polarised light identification is more often certain. The following 

 notes on the optical characters of minerals may be useful (but see 

 Rosenbusch : " Mikrosk. Physiog. der petrog. wichtigen Minera- 

 iien." 1873): 



Quartz usually shows one colour in the centre, with two or three 

 other colours around it. The colours are very bright and clear in 

 thick slices, while in thin slices they are white or grey blue. 



Orthoclase sometimes shows cross hatching. Occurring in twins, 



