THE PERCENTAGE COMPOSITIONS OF ROCKS 



279 



the first series of readings for that kind of mineral so that it may 

 not be forgotten. 



Rotate the stage of the microscope to the left until the left 

 margin of another grain of biotite hjust tangent to the cross-hair. 

 Though the point D at which the stylus rested on the outhne of 

 the last mineral measured is moved to D' (Fig. 4) the stylus in 

 the projection, of course, still rests at D. From this position 

 move upward or downward to the periphery E of the new biotite 

 grain tangent to the cross-hair and trace its outline in the same 

 direction as before. Rotate the stage until another grain is tan- 

 gent to the cross-hair, trace its outline, and so continue until all 

 of the grains of biotite in the section have been measured, and the 

 red spot again lies on the vertical cross- 

 hair. The object of rotating the stage 

 of the microscope only so far as the 

 point of tangency of each new grain is 

 to avoid confusion and the repetition 

 or omission of some fragment. Move- 

 ments up and down along the straight- 

 edge, being plus and minus, are not 

 recorded when the stylus again rests 

 at^. When all the grains of this 

 species of mineral have been measured 

 and the stylus has been returned to the 



point "^," record the planimeter reading and subtract the pre- 

 ceding reading from it. This gives the total area of the first min- 

 eral measured. If there are any inclusions in a mineral they 

 may be deducted by moving them to the point of tangency and 

 rotating the stylus around them in reverse direction. 



Begin with the red mark on the slide again at the top of the 

 vertical cross-hair and measure a second variety of mineral in 

 the same manner as before, and so on until all the minerals are 

 determined. It is usually well to measure the more pronounced 

 or least abundant minerals first, leaving for the last the most 

 abundant, least pronounced, or groundmass mineral. A slight 

 error in the other readings will change the true percentage less 



Fig. 4 



