CLASSIFICATION OF IGNEOUS ROCKS 47 



Since there are neither feldspars, feldspathoids, nor quartz, the 

 rock must belong to Class 4. 



The ratio of f erromagnesian minerals to ores is 98 : 2 ; therefore 

 the order is i. 



The ratio of pyroxene to olivine is 65 : 30 = 68 : 3 2 ; therefore the 

 rock lies along the line of FamiKes 6, 10, and 11. The ratio of 

 pyroxene to hornblende is 65:3=96:4; therefore it belongs to 

 Family i, 4, 11, or 12. Family 11 is the only one common to both 

 computations, consequently the rock number is 41 11. (To be 

 read four one eleven.) 



The rock may be plotted by drawing one line parallel to the base 

 and 30.6 above it, and another parallel to the amphibole-olivine 

 line and 66.4 from it. The intersection of the two lines locates 

 the rock. As a check, the third line, parallel to the remaining 

 side of the triangle and at a distance of 3 . o from it, should cross the 

 other two at their intersection, 



NAMES PROPOSED FOR VARIOUS FAMILIES 



On the basis of the foregoing subdivisions, nearly a thousand 

 modal analyses have been plotted and names have been given to 

 many of the families, most of them derived from plutonic rocks 

 falling at the center points. In some cases, as in the quartz-rich 

 types, family names were taken from differentiation rocks. In the 

 tabulation (pp. 12-15) there are many blank pigeonholes, owing to 

 lack of good modal descriptions. There are undoubtedly many 

 rocks in most of the families here left blank, especially in Classes 2 

 and 3, but the majority of published rock descriptions lack mineral 

 percentages, making them unavailable for classification. The writer 

 is at present engaged in measuring the components of a great 

 number of thin sections, most of them of classic rocks or of rocks 

 which have been chemically analyzed. 



Blank spaces in the tables below do not necessarily mean that 

 rocks are wanting in these pigeonholes but may indicate that none 

 falls near the center point, although, on the other hand, a solitary 

 rare rock may, in some cases, give its name to the family, even 

 though it is not at the center. 



