CLASSIFICATION OF IGNEOUS ROCKS 45 



the rock has affinities with alkaHc rocks it should be placed on the 

 fold side. 



In Class 4 the medial hnes pass through the center of gravity 

 of the triangle, consequently rocks falling on these hnes are arbi- 

 trarily moved into the famihes shown by the small arrows in 

 Figure 5. 



For classificatory purposes, it is seldom necessary to make 

 exact determinations of the mineral percentages. Unless the 

 proportions are such that the rock falls near a division line, a simple 

 inspection will answer. Thus it is usually possible to determine 

 very quickly whether the dark constituents are less than 5 per cent, 

 between 5 and 50 per cent, etc. ; whether the quartz forms more or 

 less than 5 per cent of the light constituents, and whether the ortho- 

 clase forms more than 95 per cent, between 95 and 65, between 65 

 and 35, between 35 and 5, or less than 5 per cent of the total feldspar. 

 Of course, if the rocks are to be plotted as points in the triangle, 

 more careful determinations are necessary. 



EXAMPLES 



Example i. — ^A granodiorite having the composition 



Quartz 18.0 =23.1 



Orthoclase 18.0 = 23.1= 30.0 



Andesine (AbyoAnjo) 42.0 = 53.8== 70.0 



Total quarfeloids 78.0 100 . o 100 . o 



Biotite 12.8 



Hornblende 9.0 



Magnetite i 



Titanite i 



Total mafites 22.0 



100. o 



Percentage quarfeloids = 78. The rock belongs to Class 2. 



AbyoAnjo falls between 95 and 50. The order, therefore, is 2. 



The family may be rapidly determined graphically. Plot 

 23 . 1 Qu, 23 . 1 Or, and 53 . 8 CaiVaf by drawing a horizontal Hne 

 23 . 1 above the base of the triangle, and an incHned Une, parallel 

 to the Qu-Plag hne, 23 . i from that Hne. The intersection of the 



