CLASSIFICATION OF IGNEOUS ROCKS 263 



substances which, like quartz, have a constant composition 

 present no difficulties. In other cases the composition of the 

 mineral can be ascertained from the study of the optical 

 characters ; and when this fails, it is assumed to be the same 

 as in other rocks of a similar character. 



The whole process will be found worked out in detail in 

 the case of a microgranite from the island just below Santo 

 Antonio, Rio Madeira, Brazil, in my paper on the rocks of 

 the cataracts of the River Madeira and the adjoining portions 

 of the Beni and Mamore. 1 



(2) The Calculation of the Norm 



The chemical composition having been determined, the 

 calculation of the theoretical mineral composition or norm may 

 be proceeded with. 



By dividing the percentages of the different chemical con- 

 stituents (which are, as usual in mineral analyses, given in 

 the form of oxides) by their molecular weights, the molecular 

 proportions are obtained. The molecules are then distributed 

 according to fixed rules to form the minerals of the ideal rock 

 or norm. Minerals whose composition is too complex, or which 

 are otherwise unsuitable, are excluded : for instance, the only 

 silicates containing alumina are the simple felspars (orthoclase, 

 albite, and anorthite) and the felspathoids or " lenads," as they 

 are called by the authors of this classification. 



Subject to these restrictions, the method of allotment of 

 the molecules is intended to express, as far as possible, the 

 natural affinities and preferences among the chemical con- 

 stituents of igneous rocks. The actual rules are necessarily 

 somewhat complex, and are too lengthy for repetition. They 

 will be most easily understood if I show how they are applied 

 in a particular case, that of the microgranite already referred to. 



In the following table the first column gives the results 

 of the analysis by Mr. G. S. Blake, and the second the molecular 

 proportions obtained by dividing by the molecular weights; 

 the third column shows the manner in which these molecules 

 are distributed or allotted ; and the fourth, the number of the 

 "mineral molecules" (viz. molecules representing minerals) that 

 result. 



1 Quart. Joum. Geol. Soc. vol. lxii. (1906) p. 116. 



