THE SLIDE RULE IN ROCK ANALYSES 561 



in that same mineral or to the total mineral value. Conversely, 

 the value of any radicle could be determined from the total mineral 

 value. This marked a decided step in advance, but as a complete 

 table is necessary for each mineral the disadvantages are obvious. 



Later in the same article^ Mead describes a "geologist's slide 

 rule," which is a much more efficient method of conversion, and 

 which has certainly merited a wider consideration than it has 

 apparently received. He has devised a rule for ordinary rock 

 minerals and another one for ore minerals, comprising a total of 

 nearly 100 minerals, whereby very rapid conversions and compari- 

 sons are possible, either from constituent to mineral or the reverse. 

 The rules are of white celluloid, about ten inches in diameter and 

 are exceedingly simple to manipulate. In using these rules certain 

 features became apparent to the writer: (i) a rule is good only for 

 the minerals printed thereon; (2) ordinary slide-rule computations 

 are impossible on the logarithmic scale of the rule; (3) the rule is 

 too large for ordinary pocket use and hence of limited value in 

 the field. These statements are not intended to detract in any 

 way from the value of the "geologist's slide rule," but led the 

 writer to consider a way to broaden this excellent application 

 of the logarithmic scale to geologic work. 



In a recent publication Lindgren^ gives a summary of the various 

 methods of recalculation. The scheme most favored "consists of 

 a co-ordinate system in which the abscissas represent the distance 

 from the vein which may be taken as the origin of the acting solu- 

 tions, and the ordinates represent the molecular ratios multiplied 

 by 100, except for silica, for which the scale must be reduced to 

 bring the diagram within convenient compass." This method is 

 excellent for just such cases as cited,^ but a diagram so constructed 

 is limited to one definite zone or locality and does not have the 

 range of broad comparisons and generalizations inherent in the 

 straight line diagram. 



In the conversions and comparisons of chemical analyses 

 referred to, considerable multiplication and division are necessary 



^ Ibid., p. 139. 



^Waldemar Lindgren, Mineral Deposits, 1913. Chap. xxx. 



3 F. L. Ransome, U.S. Geol. Survey Professional Paper 75. 



