USE OF THE SLIDE RULE IN THE COMPUTATION OF 

 ROCK ANALYSES 



JAMES H. HANCE 

 University of Chicago 



Certain phases of petrographic and metamorphic work require 

 the conversion of rock analyses into the corresponding approximate 

 mineralogical compositions. By ordinary methods of calculation 

 this may be rather prolonged and call for a considerable expenditure 

 of time and energy. Exact conversion is, of course, impossible, 

 especially without any preliminary data as to the minerals actually 

 present, and, in the case of such variables as the biotites, amphiboles, 

 and pyroxenes^ their chemical composition itself should be deter- 

 mined also. Nevertheless the results attainable are of sufficient 

 value to justify the procedure. 



Various methods have been suggested, each possessing more or 

 less merit and marking some advance in this direction. The use 

 of molecular ratios has been very general and has been described 

 by Kemp,^ Cross,^ and Osann.^ Harker^ calculated decimal tables 

 for the different minerals which very materially assist in converting 

 oxide values into the corresponding mineral values, but which are 

 not adapted to the reverse process which in some cases is of equal 

 importance. Mead^ devised a set of graphic tables, one for each 

 mineral, by means of which the value of any radicle in a mineral 

 might be converted into the corresponding value of any other radicle 



' Cross, Iddings, Pirsson, and Washington, Quantitative Classification of Igneous 

 Rocks, 1903, p. 145. 



^ J. F. Kemp, "The Recalculation of the Chemical Analyses of Rocks," School of 

 Mines Quarterly, XXII (1900-1901), 75. 



3 Cross, Iddings, et. al., op. cit., Part III, "Methods of Calculation." 



''A. Osann, Beitrdge zur chemischen Petrographie, I. Teil. " Molekular-Quotienten 

 zur Berechnung von Gesteinsanalysen," Stuttgart, 1903. 



5 A. Harker, private publication, Cambridge University Press, 1910. 



^W. J. Mead, "Some Geological Short-Cuts," Economic Geology, VII, No. 2 

 (March, 1912), 136. 



560 



