68 SCIENCE PROGRESS 



until the ninth subdivision (section of subsection of section of 

 subgrad !) is reached. Moreover the two kodurites analysed 

 fall into different rangs, andase and monzonase, which depend 

 on the ratio of salic alkalis to salic lime ; whereas the chief 

 difference between the two rocks depends on the composition 

 of their respective garnets. Fermor therefore concludes 1 that the 

 American Quantitative Classification fails in one instance at 

 least to exhibit the elasticity which is claimed as an advantage 

 of the system. He infers that unusual amounts of other rare 

 constituents, such as baryta, strontia, nickel oxide, etc., would 

 also affect the system adversely. It is to be remarked, how- 

 ever, that a factor (such as MnO) which is of very minor 

 importance in the vast majority of igneous rocks, should also 

 be of minor importance in classification, notwithstanding its 

 occasional abundance in an abnormal and rare series of rocks. 

 Essentially Fermor demands that a classificatory factor should 

 vary in importance according to its significance in the rocks 

 which it is proposed to classify. But no consistent logical 

 classification could be erected on such a basis. Classification 

 must be based on the factors dominant, in significance and most 

 frequently also in mass, in the whole series of igneous rocks ; 

 and it is these factors which are used in the American 

 Quantitative Classification, and which will have to be used in 

 some form or other in all quantitative systems. 



The American Quantitative Classification itself is not ex- 

 empt from a similar error ; and to this is due an asymmetry 

 of the system which does not yet appear to have been pointed 

 out. I refer to the radically different method of forming the 

 orders in Classes IV. and V., as compared with Classes I., II., 

 and III. In the latter the orders are based on the ratios of 

 quartz or lenads to felspars ; in the former on the ratios of 

 pyroxenes plus olivine to the mitic minerals (magnetite, 

 ilmenite, haematite, titanite, etc.). This introduces a most con- 

 fusing break between Classes III. and IV., renders the com- 

 partments incommensurate on either side of the partition, and 

 makes the; tracing of a petrographic series from Classes I., II., 

 and III. into Classes IV. and V. almost impossible. Employing 

 the figures already utilised, the pigeon-holes in Classes I., II., 



1 L. L. Fermor, "The Systematic Position of the Kodurite Series, especially 

 with reference to the Quantitative Classification," Records Geo/. Surv. India, xlii. 

 1912, p. 210. 



