PROBLEMS OF PETROGRAPHIC CLASSIFICATION 799 



eighth place in the Quantitative System seems quite appropriate. 

 It is certainly plain that a scheme recognizing manganese in the 

 third place would, for example, not serve to bring out the actual 

 close relationship which exists between the Boirani and Kotakarra 

 kodurites, for the former has only 0.98 per cent MnO. Fermor 

 himself points out that, considering manganese as the character- 

 istic thing about kodurite; the Boirani rock is classed with it simply 

 for convenience. It is modal garnet and orthoclase or normative 

 anorthite and orthoclase which bring the rocks together. 



Fermor expresses the opinion that the Quantitative System does 

 not possess either the elasticity or the comprehensiveness which 

 the writer has said should be found in a satisfactory classification. 

 If by elasticity is meant the possibility of transposing certain 

 fundamental parts of the structure at will, according to the varying 

 prominence of certain constituents in different rocks, the first part 

 of this comment is true, but it may be answered that no real 

 system can be so constructed. The section of subgrad necessary 

 to express the manganiferous character of kodurite is, however, 

 not an appendix, as Fermor considers it. It is an extreme division 

 of the system connected appropriately with the rest in a manner 

 provided for in the original publication. 



"Kodurose," as a subgrad magmatic name, does not itself 

 express all the chemical relations involved, but it is no more neces- 

 sary to go over all the steps of the classification by which this 

 division has been reached, whenever it is referred to, than it is in 

 using the specific or varietal names of animals or plants. It may 

 be pointed out that where the chemical relations of igneous rocks 

 are in question, the new symbols for expressing quantitative classi- 

 fication do show the whole systematic position far better than does 

 any existing scheme for concise statement in zoology or botany.^ 

 "Kodurose" does express the important relationships of such a 

 manganese rock to other rocks in all important respects — when the 

 system is understood. 



Fermor considers the Quantitative System unable to deal with 

 rocks containing large amounts of oxides of manganese, nickel, 



I Cross, Iddings, Pirsson, and Washington, "Modifications of the Quantitative 

 Classification," etc., Jour. Geol., XX (1912), 553-57- 



