[graham] specific GRAVITY OF MINERAL FRAGMENTS 53 



The specific gravities of several different substances may be 

 determined, if desired, during the course of a single experiment. The 

 method is also especially convenient in determining the character 

 of theplagioclase feldspar in rocks. For this purpose, optical methods, 

 such as a measurement of extinction angles, or of the mean refractive 

 index, are perhaps more commonly employed, but the plagioclase in a 

 rock being usually not all of the same composition, there is always the 

 danger that the fragments examined do not represent the principal 

 feldspar present. By carrying out a specific gravity determination 

 as described above, however, it is possible to arrive at a knowledge of 

 the composition, not only of the principal feldspar, but also of the most 

 acid (lightest) and most basic (heaviest) varieties present. 



Geological Department, 

 McGill University, 

 Montreal, Que. 



