S68 JAMES H. HANCE 



As is true in many other instances, description is much slower 

 and more tedious than manipulation, and the operations herein 

 outlined may be performed easily in a few moments after one is 

 accustomed to the use of the slide rule. To a person unfamiliar 

 with the rule the question of placing the decimal point at once 

 suggests itself. Used with the accompanying tables, the percentage 

 values give all the assistance needed on this point and render the 

 correct interpretation here more facile even than with the "geolo- 

 gist's slide rule." The values obtained should bear the same rela- 

 tions to each other as do the corresponding percentage values in 

 the tables. For other operations the reader unaccustomed to slide- 

 rule use is referred to various directions which are furnished by the 

 manufacturers. 



The accompanying tables are believed to be complete enough 

 for most metamorphic, petrographic, and economic uses, but can 

 be augmented to suit special needs. The percentages of a few 

 oxides are included, as .these are desirable in many cases. Any 

 arrangement of minerals in such a table can be made to suit indi- 

 vidual needs, and such tables mounted separately or back to back 

 and covered with gelatin will be found an excellent form for field 

 use, if such is desired. 



