CLASSIFICATION OF IGNEOUS ROCKS 43 



GLASS 



Glass must be computed from an analysis. One can usually 

 surmise its composition from the character of the phenocrysts and 

 the appearance of the rock as a whole. When undetermined, the 

 rock must be given a tentative name, such as hyaline-rhyolite, etc. 



RULES FOR COMPUTING ROCKS FROM THEIR MODES 



1. The sum of the minerals in the mode should be 100=1=0.5. 

 If less it should be recalculated to 100. The sum of the leucocrates 

 (quarfeloids plus auxihary minerals) determines the class. 



Class I. Leucocrates form more than 95 per cent of the total component. 



Class 2. Leucocrates between 95 (inclusive) and 50 per cent. 



Class 3. Leucocrates between 50 (inclusive) and 5 per cent. 



Class 4. Leucocrates between 5 (inclusive) and o per cent. 



2. Determine the orders in Classes 1,2, and 3 directly from the 

 Ab-An ratio in the plagioclase. 



Order i. AbiooAno to AbgsAnj. 



Order 2. AbgsAnj (inclusive) to AbjAnso. 



Order 3. AbsoAnjo (inclusive) to AbjAugs. 



Order 4. AbsAngs (inclusive) to AboAnioo. 



In Class 4 the orders are determined by the percentage of " ores." 

 Reduce the sum of biotite, olivine, pyribole, and "ores" (including 

 cassiterite, chromite, etc.) to 100, dropping the minor mafites, 

 apatite, garnet, perofskite, any small amount of quarfeloids, etc. 

 The percentage of "ores" determines the order. 



Order i. o to 5 per cent "ores." 



Order 2. 5 (inclusive) to 50 per cent "ores." 



Order 3. 50 (inclusive) to 95 per cent "ores." 



Order 4. 95 (inclusive) to 100 per cent "ores." 



3. Determine the family. In Classes 1,2, and 3, first recalculate 

 the quarfeloids to 100. The amount of quartz (or feldspathoid) 

 thus determined immediately locates a row of five horizontal 

 pigeonholes, in one of which the rock belongs. Recalculate' Kf 

 plus plagioclase to 100 and determine the proper point on the 



' It is immaterial whether the orthoclase-plagioclase ratio is taken from the 

 original values or from those reduced as quarfeloids to 100. The results are naturally 

 the same. 



