8o6 WHITMAN CROSS 



relatively ''largely developed." Many phonolites, foyaites, and 

 nephelite syenites carry but lo per cent or even less of feldspathoid. 

 There is no indication of the minimum amount of nephelite or 

 leucite which Hatch would consider as placing a rock in the "Felds- 

 pathoid Series." 



In subdividing the feldspathic rocks still farther, Hatch intro- 

 duces as a direct quantitative factor the relative percentages of 

 alkali feldspars as against plagioclase, and of soda as contrasted 

 with potash feldspar. This is certainly a step in the right direction, 

 but as it comes after the irrational silica distinction, it loses much 

 of the value it might have in bringing allied rocks together. 



It is to be hoped that Fermor will succeed in establishing the 

 origin and chemical character of the interesting rocks called the 

 Kodurite Series. If they possess the characters assumed for them, 

 it appears to me that the Quantitative System will show — -so far 

 as it professes to go, that is, in expressing chemical characters — • 

 both their general chemical relations to other rocks and their unique 

 special features. 



The use of factors of natural occurrence in the classification of 

 igneous rocks does not necessarily make the system natural. The 

 system of Hatch,, like many others of allied construction, does not 

 seem to me to use natural factors in a rational way And as actual 

 characters of the rocks are used with unnecessary arbitrariness the 

 right of his and allied systems to be termed natural is open to 

 question. 



The systematic treatment of igneous rocks, with all their widely 

 varying important characters, is a matter of great difficulty, and 

 a long time may elapse before petrographers come to a common 

 point of view. It is in the hope of contributing, however slightly, 

 to this understanding, that this discussion has been written. 



