in Rock-sections by maximum .Birefringence. 263 



known colors rather than the transition tones. If anything is 

 sacrificed to accuracy by this method, it is more than compen- 

 sated for by simplicity and convenience in use. 



In using this diagram (p. 265) for the determination of an un- 

 known mineral, the thickness of the rock-section is first deter- 

 mined. This is done by taking some well-known mineral, such as 

 quartz, for instance, which furnishes a number of sections in the 

 slide and observing the highest color given by any of them. The 

 maximum birefringence for quartz is 0*009 as seen in the table 

 and by following this line down toward the left-hand lower 

 corner until it intersects the vertical line of the highest inter- 

 ference-color observed, and by then following out to the edge on 

 a horizontal line the thickness of the section can be told. The 

 numbers represent hundredths of millimeters.- 



For the determination of an unknown mineral the highest 

 color given in numerous sections is observed and the thickness 

 having been determined as above, by means of the diagonals 

 the numerical value is noted which corresponds to the given 

 color in a section of the determined thickness. The maximum 

 birefringence of the unknown mineral having been determined 

 the table of birefringences is referred to and the mineral 

 usually found to be one of a group of several having approxi- 

 mately the same values. Which particular one it may be, is in 

 the vast majority of cases readily told by its comparative 

 refractive index in plain light, cleavage, color and other optical 

 properties. 



Thus, for example, in a slide having numerous quartz sec- 

 tions the highest color shown is a pale yellow. The bire- 

 fringence of quartz is 0*009. The diagonal 0*009 crosses the 

 line of pale yellow at about 0*035 which gives the thickness of 

 the section. In the slide are numerous grains of an unknown 

 mineral, colorless, of strong refraction and whose highest color, 

 given by several of them, is an orange-red of the second order. 

 But there passes through the intersection of the thickness 

 0*035 and the line of orange-red of the second order the diago- 

 nal 0*029 which is the approximate value of the birefringence 

 of the mineral. Referringto the table this is found to approxi- 

 mate to that of allanite, chondrodite, diopside cancrinite and 

 tremolite. The refraction, color, cleavage, etc., show at once 

 that of these, diopside is the only one which corresponds and 

 thus the mineral is determined. 



Sheffield Petrographical Laboratory, 



Tale University, ISTe.w Haven, Conn., April, 1900. 



