AN IMPROVED RECORDING MICROMETER 

 FOR ROCK ANALYSIS 



CHESTER K. WENTWORTH 



University of Iowa, Iowa City, Iowa 



Numerous methods have been devised for the quantitative 

 estimation of the mineral composition of rocks by measurement of 

 the minerals appearing in thin sections. These have been described 

 in detail elsewhere,^ and it is not the writer's purpose to consider 

 their relative merits at length. All these methods involve the 

 measurement of areas either directly or by weighing dissected rock 

 patterns, or of intercepts of the several minerals along certain selected 

 lines. In any case, the number of measurements must be large in 

 order to overcome the errors due to variation in grain size and 

 irregularity of distribution of the minerals, and the process by any 

 one of the methods is a somewhat laborious one. Among the several 

 methods, that of Rosiwal seems to combine the requisites of sim- 

 plicity and relative accuracy. This method has been the subject of 

 careful examination by Johannsen and Stephenson^ from the 

 empirical standpoint, and by Lincoln and Rietz^ from the theo- 

 retical as well as the experimental point of view. The work of these 

 investigators proves adequately the validity of the method, and 

 that of Lincoln and Rietz is valuable particularly in offering the 

 means of defining somewhat accurately the requisite number of 

 measurements for a given degree of precision in results obtained 

 under various conditions. 



' A. Johannsen, Manual of Petrographic Methods (New York, 1914), PP- 290-92, 

 and "A Planimeter Method for the Determination of the Percentage Compositions 

 of Rocks," Jour. GeoL, Vol. XXVII (1919), pp. 276-85; A. Hohnes, Petrographic 

 Methods and Calculations (London, 1921), pp. 310-22; J. Hirschwald, Eandbuch der 

 bautechnischen GesteinsprUfung (Berlin, 1912), pp. 146-47J 163-72; A. Rosiwal, "tjber 

 geometrische Gesteinsanalysen," Verh. der k. k. geolog. Reichanstalt (Wien, 1898), 

 No. 5. 



*A. Johannsen and E. A. Stephenson, "The Rosiwal Method for Minerals," 

 Jour. GeoL, Vol. XXVII (1919), pp. 212-20. 



3F. C. Lmcoln and H. L. Rietz, "The Determination of the Relative Volmnes of 

 the Components of Rocks by Mensuration Methods," Econ. GeoL, Vol. VIII, pp. 120-39. 



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