46 ALBERT JOEANNSEN 



two in Family 9 determines the position of the rock. As a check, 

 the point must also lie 53 . 8 from the Kf-Qu Kne. 



To compute the family analytically: From the presence of 

 23 . 1 per cent quartz, the family must lie between numbers 6 and 10, 

 since there is more than 5 per cent and less than 50 per cent quartz. 

 Further, the ratio of Or to CaiVaf is 30 to 70, and since the orthoclase 

 is between 5 and 35 per cent the family number is 9. 



The rock number, therefore, is 229, that is, Class 2, Order 2, 

 Family 9. (The number is to be read two two nine.) 



Example 2. — A nephelite-monzonite with 



Kf 21.5 = 39.0 



Oligoclase (AbpzAng) 33 . 5 = 61 .0 



Total feldspar 55 .0 100. o 



Neph 27.5 



Sodal 8.5 



Total feldspathoids 36.0 



Total quarfeloids 91. o 



Aeg.-aug 50 



Biot 2.5 



Access 1.5 



9.0 



Quarfeloid ratio 91, Class 2. 

 AbgzAng, Order 2. 



Folds to feldspars = 36 155 = 39. 5: 60. 5. Between Families 21 

 and 25. 



Kf to CaiV"af = 39. 0:61.0. Family 23, 



Rock number is 2223. (To be read two two twenty- three.) 



Example 3. — A Iherzolite with 



Augite ^^'°\= 66 ± 



Hypersthene 20.0/ 



Olivine 30 . o = 30 . 6 



Hornblende 30= 30 



Magnetite 2.0 100. o 



100. o 



