562 



JAMES H. HANCE 



in addition to that obviated by the "geologist's sKde rule." For 

 this work slide-rule accuracy is sufficient. In extended studies 

 of this nature the use of the rule may save from 50 to 90 per cent 

 of the time required for such calculations. Hence, if these conver- 

 sions could be simply accomplished on an ordinary slide rule, con- 

 siderable time and energy would be saved and the rule might then 

 attain the same merited popularity with geologists that it has 

 already won in engineering lines. At the same time its compact 

 nature lends itself more readily to field use. 



For these reasons the writer submits the accompanying tables 

 (Tables I and II) of mineral percentage compositions, with a method 

 of use applicable to any ordinary slide rule, which he trusts may 

 meet the foregoing conditions successfully. In addition, conver- 

 sion into molecular values by means of tables such as given by 

 Osann^ can be so much more quickly accomplished on a slide rule 

 than by ordinary arithmetical processes. 



*R. A. Daly, Igneous Rocks and Their Origin, 1914, p. 34, No. 97 (alaskite). 

 t Ihid., p. 32, No. go (leucitite). 



These tables contain two sets of minerals, one the more com- 

 mon of the rock-forming variety, the other the more important 

 ore minerals. Precise mineral composition is, of course, only 

 theoretically attained, but the formulae given are taken from 

 standard texts^ and represent generally accepted values. An 



^ A. Osann, Beitrage zur Chemischen Petrographie, I. Teil, "Molekular-Quotienten 

 zur Berechnung' von Gesteinsanalysen," Stuttgart, 1913. 



" E. S. Dana, Textbook of Mineralogy; Moses and Parsons, Mineralogy, Crystal- 

 lography and Blowpipe Analysis; F. W. Clarke, "Data of Geochemistry," U.S. 

 Geological Survey Bulletin 4gi; Albert Johannsen, Determination of Rock-forming 

 Minerals in Thin Sections. 



