PROBLEMS OF PETROGRAPHIC CLASSIFICATION 805 



SiOa 63.09 has 22.9 per cent normative quartz; the quartz diorite 

 of SiOa 57.41 has 11. 5 per cent normative quartz. Many rocks 

 are nearly normative in their quartz contents and some carry more 

 than the normative amount. 



The ultrabasic rocks furnish further evidence, if such is neces- 

 sary, that a division of rocks by sihca percentage is without reason 

 and without object so far as any relation between chemical and 

 mineral composition is concerned. The line of 52 per cent sihca, 

 established by Hatch between intermediate and basic rocks, is 

 passed by the silica of some rocks placed by him in the ultrabasic 

 group, e.g., websterite, one analysis of which, cited by him, has 

 53.25 per cent SiOz. 



The norms of many of Hatch's ultrabasic rocks contain 20 per 

 cent or more of feldspar and lenad. And while such amounts are 

 not common in the mode of the plutonic rocks in question, those of 

 the surface or hypabyssal forms of equivalent chemical composi- 

 tion are often notably feldspathic. This well-known variation 

 in mineral composition, due to physical conditions, must be con- 

 sidered in any adequate systematic treatment of the more basic 

 or "ultrabasic" rocks. 



After the arbitrary division on silica percentage, Hatch intro- 

 duces the character of the feldspars and feldspathoids as the factor 

 for the next subdivision. It is his evident intent to emphasize the 

 quantitative element in many places, but this is often by use of 

 the vague terms "predominant," "largely developed," and "sub- 

 ordinate." The meaning attached to such terms is often highly 

 subjective, as appears most striking in the case of the "Felds- 

 pathoid Series," which is defined as one "in which nepheline, 

 sodahte, or allied feldspathoid is largely developed, and feldspar 

 subordinate."^ But Hatch cites, as illustrating the "Feldspathoid 

 Series," the average of nine typical nepheline syenites, given by 

 Brogger, and the norm calculated from this, which must approxi- 

 mate closely to the mode, gives feldspars 69.11 per cent, and 

 nephehte 20.3 per cent. That is, the feldspars, which must be 

 more than three times as abundant as nephelite in a rock of the 

 cited composition, are declared "subordinate," and the nephelite 



' Science Progress, p. 3. 



