30 NEW YORK STATE MUSEUM 



Many years ago the late George W. Hawes^ noticed in slides of 

 these rocks some feldspars which failed to afford the twinning stria- 

 tions yet which he suspected of being plagioclase. Analytical tests 

 demonstrated that they were. The same untwinned character may 

 reappear so that the observer must be on his guard, but it is also 

 true that a chance section parallel to the twinning plane would 

 also be without the striations. 



The analyses demonstrate the presence of potash quite without 

 exception. The extreme rarity of biotite in the localities where 

 the specimens taken for analysis were collected make it practically 

 certain that the potash is in the orthoclase molecule and that this 

 feldspar is in the rocks up to 5 per cent or over. It would also 

 yield untwinned feldspar, which could only be distinguished from 

 plagioclase by refined optical tests. 



The analyses prove that quartz is at times present in amounts 

 even reaching 8 per cent. The observer would need to exercise 

 care not to overlook this mineral, yet despite the rather large per- 

 centage indicated by the recasting of the analyses it is rare to detect 

 it in the slides. It is possible that it may be separated in part 

 during the process of saussuritization, and be so finely divided in 

 this indefinite, cloudy mass as to escape notice. 



Microscopic study has shown that the commonest and most 

 v/idely distributed pyroxenic component is a pale green variety, no 

 doubt near diopside if not actually this molecule. The relatively 

 high percentage of lime in the analyses is sufficient to more than 

 satisfy the anorthite molecule and still leave an excess for the 

 pyroxene, while the relatively low magnesia and iron serve to keep 

 the hypersthene molecule somewhat in the background. Hyper- 

 sthene is, however, abundant and widely distributed and as soon as 

 the percentages of magnesia and ferrous iron rise and the anortho- 

 sites develop larger percentages of the pyroxenic components^ the 

 hypersthene becomes prominent. In the more coarsely crystalline 

 and pegmatitic phases the hypersthene assumes a coarseness of 

 crystallization which combined with its easily recognized bronze lus- 

 ter, makes it catch the eye of the observer and convince him of its 

 presence. In the typical anorthosites the pyroxenic components 

 are smaller in size than the feldspars and are packed in between 

 the latter. Actual contact between the two, and especially in the 



1 Hawes, George W. On the Determination of Feldspar in thin sec- 

 tions of Rocks. U. S. Nat. Miis. Proc. 1882. 4:134-36. 



