IGNEOUS ROCK-MAGMAS AS SOLUTIONS 247 



may build up composite crystals, as in the case of perthite, 

 which is, according to Vogt's researches, a eutectic intergrowth 

 of a potash- and a soda-felspar in the proportions of about 2 to 3. 

 To Professor Vogt, of Christiania, belongs the credit of the 

 first comprehensive essay towards bringing the crystallisation 

 of igneous rock-magmas definitely under the known laws of 

 solutions. The starting-point of his work is a chemical, physical, 

 and microscopical study of a large series of silicate-slags, more 

 or less closely comparable in composition with igneous rocks. 1 

 A large body of information relative to these has been accumu- 

 lated by Akerman, Vogt, and others, and the use made of these 

 data affords a striking illustration of the service which can be 

 rendered to pure science by researches which were, in their 

 first intention, technological. 



The minerals of these slags include olivine and fayalite, 

 monoclinic and rhombic pyroxenes, wollastonite and pseudo- 

 wollastonite, lime-felspars, melilite, spinel, magnetite, apatite, 

 and others. Setting aside a few which are not found as rock- 

 forming minerals, they are the ordinary constituents of the more 

 basic igneous rocks. The alkali-felspars, quartz, muscovite, 

 hornblende, and some less important minerals do not occur in 

 slags, and cannot be artificially reproduced without the aid of 

 fluxes. The application to these minerals of the results obtained 

 from the study of slags is therefore a matter of inference rather 

 than of direct demonstration ; but petrographical considerations, 

 such as those concerning micrographic and perthitic inter- 

 growths, come to reinforce powerfully the hypothesis which 

 embraces all the rock-forming minerals under the same laws. 



Vogt's principal memoir (1903-4) consists of two parts. The 

 first part is essentially an investigation of the order of crystal- 

 lisation as dependent on the relative amounts of the several 

 constituents. Microscopical examination shows what minerals 



' See especially Die Silikatschmelzlosungen {Vidensk.-Selsk. Skrifter, Math.- 

 Naturv. Klasse, 1903, No. 8, and 1904, No. 1). For Vogt's earlier writings on slags 

 see Studier over Slugger (Bihang til k. Svenska Vet.-Akad. Handl., vol. ix. 1884) ; 

 Om. Sluggers . . . {Jernkontorets A?inaler, 1885) ; Beitrdge zur Kenntniss der 

 Gesetze der Mineralbildung in Sch?nelzmassen {Arch, for Math, og Naturvid., vols, 

 xiii., xiv., 1888-90). For a general summary of his views see Die Theorie der 

 Silikatschmelzlosungeti {Bericht des V. Intern. Ko?igr. angew. Chemie zu Berlin, 

 1903, Sekt. III. A., vol. ii. pp. 70-90). For further development of some parts of 

 the theory, see " Physikalisch-chemische Gesetze der Krystallisationsfolge in 

 Eruptivgesteinen," Tscherm. Min. Petr. Mitth., (2) vol. xxiv. pp. 437-542, and 

 vol. xxv. pp. 361-412 (1906). 



