CLA SSIFICA TION OF IGNE OUS RO CKS 5 7 9 



I. In which A is alone present or is extremely abundant. 

 II. In which A dominates over B. 



III. In which both A and B are present in equal or nearly equal amounts. 



IV. In which B dominates over A. 



V. In which B is alone present or is extremely abundant. 



The factor which is extreme or dominant over the other, will 

 be spoken of as preponderant, the other being subordinate. 



To indicate whether one of a pair of factors is present in 



greater or less amount than one-eighth of the combined pair of 



A i 

 factors, that is, -= ^ - , we have adopted the terms notable and 



negligible. The former means that it is present in greater 

 amount, and the latter that it is present in less amount, than 

 that proportion. 



It is to be noted that the smallest amount of any factor which 

 we recognize as ?iotable in a particular division of the system is 

 one-eighth of the combined pair of factors, not necessarily one- 

 eighth of the whole rock, since the pair of factors may be a 

 fraction of the entire rock. 



This fivefold method of subdivision will be further applied 

 to that factor of a pair which is extreme or domina?it over the 

 other, or, in the case where both are present in equal amounts, 

 it will be applied first to that one which is deemed to be of 

 the greater importance, and subsequently, when necessary, to 

 the less important factor. 



In the case of one factor which is subordinate to another, 



but not negligible, ( < - > - 1 , such a manifold subdivision is not 



advisable, since one-fourth of the factor in question would be 

 less than one-eighth of the pair, and hence negligible. In this 

 case we have adopted a threefold division, by retaining the 



central division of equal proportions I <-> -I , and uniting the 



dominant and extreme divisions. In these divisions there is one 



where two factors are equal or nearly equal (<-> -1 , and two 



where one factor dominates or is extreme (> -J. Strictly, the 



