THE SLIDE RULE IN ROCK ANALYSES 



563 



uJ Q 

 _|Z 





average value for specific gravity is given instead of the range 

 usually stated, as this average is accurate enough for such com- 

 putations, and in fact is more accurate than the reconstructed 

 analyses. This is due to the fact that the rocks of unhke mineral 

 composition may have identical chemical composition, and also 

 due to the range in chemical composition of such minerals as the 

 biotites, amphiboles, pyroxenes, garnets, spinels, etc. The use 

 of the tables may be explained most easily by taking two rock 

 analyses and performing two conversions such as commonly con- 

 front the petrographer. 



u °8 



LEFT SCALE A & B 



-JL 



ii i|iiii[ji ii |i i ii[in i |i ii i[|ii|i ii i| i i i i|i ii i|ji i |iiii| i ji| i i i |i 



C & D SCALES 



Fig. I 



The ordinary slide rule (see Fig. i) has four graduated logarith- 

 mic scales, two on the immovable portion and two on the slide. 

 For convenience these are referred to as "A," "B," "C," and "D," 

 beginning with the upper one and reading downward. For brevity's 

 sake the vertical line on the glass slide will be termed "E." A and 

 B are duplicates, as are C and D. In using the mineralogical tables 

 the scales C and D only are needed. First we shall proceed to 

 reconstruct albite in Analysis I. The chemical analysis shows the 

 presence of 3.55 per cent NazO. In Table I the percentage com- 

 position of albite is found to be 



Na.O— 11.8; AlA— 19 s; SiO— 68.8 



Set "E" on 118 on ''D" and move "C" until 355 on this scale is 

 under "E. " Then if the corresponding amount of albite is desired. 



