A PLANIMETER METHOD FOR THE DETERMINATION 

 OF THE PERCENTAGE COMPOSITIONS OF ROCKS 



ALBERT JOHANNSEN 



University of Chicago 



The great objection to the Rosiwal method for determining 

 the composition of rocks hes in the nerve-racking necessity of 

 continually reading the stage-micrometer, and the length of time 

 required for the process. In a recent paper in the Journal of 

 Geology, Shand^ described a mechanical stage by the use of which 

 very few readings are necessary. An instrument of this kind 

 undoubtedly would greatly reduce the labor of computing the 

 composition of a rock and tend to make the method more popular. 

 Unfortunately few laboratories are likely to have such a stage; 

 but with the equipment on hand in most institutions, measure- 

 ments more rapid and possibly more accurate may be made. 



Many years ago Delesse^ determined the compositions of rocks 

 by tracing, on oiled paper, the outline of each component as shown 

 in polished slabs. After coloring his drawing he pasted it on tin 

 foil, cut apart and sorted the different components, soaked off the 

 paper, and weighed the tin foil. Later, SoUas^ applied the method 

 to thin sections by using a camera lucida, and Joly* used photo- 

 micrographs. Since Rosiwal^ introduced his method of linear 

 measurements in 1898, it has been the system commonly used. 



' S. J. Shand, "A Recording Micrometer for Geometrical Rock Analysis," Jour. 

 Geol., XXIV (1916), 394-401. 



^ A. Delesse, "Precede mechanique pour determiner la composition des roches,'' 

 Comptes Rendus, XXV (1847), 544~"4S; see also article, "Procede mechanique pour 

 determiner la composition des roches," Ann. d. Mines, XIII (1848), 379-88. 



3 W. J. Sollas, "Contributions to a Knowledge of the Granites of Leinster," 

 Trans. Roy. Irish Acad., Diihlin, XXIX (1887-92), 471-73. 



''J. Joly, "The Petrological Examination of Paving-Sets," Proc. Roy. Dublin 

 Soc, X (1903-5), 62-92. 



s August Rosiwal, "tJber geometrische Gesteinsanalysen," Verh. d. k. k. geol. 

 ReichsansL, Wein, 1898, p. 143. 



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