CLA SSIFICA TION OF IGNEO US RO CKS 66 1 



Accurate quantitative determination of the mineral compo- 

 nents of rocks by optical methods is difficult with coarse-grained 

 and coarsely porphyritic rocks as well as with extremely fine- 

 grained ones. When the rock contains large crystals a com- 

 paratively large area of it must be measured to obtain correct 

 proportions of the component minerals. A few thin sections 

 are not adequate. The measurements must be made mega- 

 scopically. The same is true when there are large phenocrysts. 

 A sufficiently large area of surface must be measured to furnish 

 a correct estimate of the relative proportions of the several kinds 

 of phenocrysts and the groundmass. Subsequently the ground- 

 mass may be studied and measured with a microscope and the 

 two sets of measurements combined. 



In very fine-grained rocks, where the kinds of minerals com- 

 posing them can all be identified, the accuracy of measurements 

 of the diameters of crystals with a microscope is affected by the 

 overlapping of crystals within the section, and it is found by 

 experience that the amount of the colored crystals is overesti- 

 mated, while that of the colorless ones is underestimated. This 

 is particularly the case where the thickness of crystals is a frac- 

 tion of the thickness of the rock section, as with microlites and 

 minute inclusions. It will be necessary to determine corrections 

 to be applied in such cases by working on microcrystalline or 

 microlitic rocks whose chemical composition has been deter- 

 mined. 



If all of the minerals actually present in a holocrystalline 

 rock are standard minerals, salic or femic, there may still be 

 uncertainty as to the norm, since the proportions of the standard 

 minerals actually developed may not accord with those consti- 

 tuting the norm. In all cases it is necessary to calculate the 

 norm from the actual mineral composition quantitatively deter- 

 mined by estimating the chemical composition of the rock from 

 that of each of its component minerals, and from this analy- 

 sis deducing the norm as in the first method described. 



This involves the determination of the chemical composition 

 of the actual minerals present in the rock. For a certain num- 



