WARREN. — ALKALI-GRANITES AND PORPHYRIES. 229 



the average of which are, quartz 30.6, feldspar 60.6, dark minerals 

 9.4. 



The albite and microcline in the microperthite are almost exactly 

 equal, which is also in agreement with crude microscopic estimates. 

 The Na2 Fe2 Si40i2 (aegirite and corresponding hornblende) molecule 

 predominates but the ferrous or ferro-ferri compounds are prominent, 

 a fact to be expected from the known composition of the Riebeckite 

 in the pegmatitic facies where these molecules exceed the Na2 Fe2 Si40i2. 



Comparing the analyses of other Riebeckite granites given in col- 

 umns 6, 7, and 8 with each other and with that of the Quincy granite, 

 we note that all are characterized by exceedingly low MgO and CaO, 

 by high iron content in both ferrous and ferric iron, and by high 

 alkalies. The soda predominates, molecularly. With the exception 

 of No. 6, in which the soda is greatly in excess, they show very nearly 

 equal percentages of the two alkalies, though in view of the considera- 

 ble variation in alumina this perhaps has little significance. An 

 approximate calculation of the mineral composition of each is given 

 below and shows a great variation in the relative percentages of the 

 minerals which are in each case of essentially the same type. 



100.0 100.0 100.0 100.0 



Comparison of the three analyses of the Quincy granite with each 

 other shows that there is some variation in the amounts of different 

 oxides, and therefore, in the minerals. These variations were ex- 

 pected from facts revealed by the study of the thin-sections. Thus 

 the granite of the Hitchcock quarry (1) was richer in aegirite and 

 showed less of the riebeckite in the form of microclites scattered 

 through the feldspar. The ferric-iron should therefore be relatively 

 higher than in the dark granite from the Reinhalter quarry, in which is 

 very abundant riebeckite (a hornblende rich in ferrous iron) both in 

 large crystals and distributed through the feldspars. The samples 

 came from localities separated by some distances and different eleva- 

 tions and are representative of the granite as a whole. Taken jointly 



