CORRODED CRYSTALS 1071 



When convergent light is required the slide on the stage and 

 either C4 or H are pushed in, and the eyepiece covered with the 

 analyser B'. The optical figures of the crystal then appear with 

 almost ideal clearness. If this simple method is compared with that 

 previously in use, the superiority of the instrument will be im- 

 mediately recognised. It is in fact the most perfect petrological 

 microscope yet issued, and is one which will suit equally the minera- 

 logical and petrological student. 



The microscopical investigation of rock sections has almost re- 

 volutionised petrology. Although the geologist has no difficulty in 

 determining by his unaided eye wtth the use of simple chemical tests 

 the mineral components of rocks of coarse texture, the case is 

 different with those of extremely fine grain ; still more with such as 

 present an apparently homogeneous, compact, or glassy character. 

 The study reveals facts of the most striking significance, and wel- 

 come light has been thrown upon the question of the order and 

 method of formation of rock constituents. 1 



The material which issues from a volcano during an eruption 

 is rarely in a state of complete fusion. In most cases it contains 

 crystals and parts of crystals which have formed before the arrival 

 of the fluid mass at the surface of the earth. Such crystals are 

 usually of large size and can generally be recognised with the naked 

 eye. But sometimes these have undergone other changes before the 

 final consolidation of the rock. They may have been formed under high 

 pressure, for the pressure lowers the melting-point of most substances. 

 Accordingly, as the pressure is relieved upon the lava getting at or 

 near the surface, the crystals which are floating in the fused mass 

 at the time are liable to become corroded or redissolved. Again, some 

 subterranean change may produce a distinct rise in the temperature 

 of the mass, or an access of heated water may increase the solvent 

 power of the molten portion. Instances of corrosion from one or 

 more of these causes are numerous. The quartzes of the quartz- 

 through the substance and its outside measure, the milled head being divided to 50, 

 and each division equalling one thousandth of a millimetre. A wheel of small aper- 

 tures is fitted to the upper Bertrand lens of the microscope for the purpose of show- 

 ing optical pictures in minute crystals of various sizes. 



1 The reader is referred to the following works treating of the microscopical charac- 

 ters of minerals and rocks : F. Fouque* et Michel Le*vy, Mineralogie micrographique, 

 Paris, 1878 ; E. Hussak, Anleitung zumBestimmendergesteinsbildenden Mineralien, 

 Leipzig, 1885; E. Kalkowsky, Elements der Lithologie, Heidelberg, 1886; A. V. 

 Lasaulx, Elemente der Petrographie, Bonn, 1875, and Einfiihrung in die Gtsteins- 

 lehre, Breslau, 1886 (also edition in French) ; Levy et Lacroix, Les Mineraux des 

 Bodies, Paris, 1888 ; F. H. Eosenbusch, Mikroskopische Physiographic, 2nd edition, 

 vol. i. ' Die Mineralien ' (translated into English by Iddings), vol. ii. ' Die massigen 

 Gesteine ; ' Hulfstabellen zur mikroskopischen Mineralbestimmung in Gesteinen 

 (translated into English by F. H. Hatch) ; and Elemente der Gesteinlehre, 1898 ; 

 F. Rutley, The Studij of Rocks, 3rd edition, 1884, and Bock-forming Minerals, 1888 ; 

 J. J. H. Teall, British Petrography, 1888 ; F. Zirkel, Lehrbuch der Petrographie, 

 2 vols. 2nd edition, 1893 ; Basalt gesteine, Bonn, 1870 ; Die mikroskopische Beschaf- 

 fenheit der Mineralien und Gesteine, Leipzig, 1873 ; Microscopical Petrography 

 (U.S. Geol. Exploration of 40th parallel), Washington, 1876 ; A. Harker, Petrology 

 for Students, 1895 (1st edition). The English student will find much valuable infor- 

 mation and useful directions in G. A. J. Cole's Aids to Practical Geology. But the 

 literature is now so voluminous that it is practically impossible to give anything like 

 a complete list ; for important papers will be found in almost every periodical deal- 

 ing with geology, among which those published in the United States must not be 

 forgotten. 



