144 PROCEEDINGS OF THE AMERICAN ACADEMY. 



characteristic of both is the great overlapping of the periods of 

 crystalHzation of the minerals. In fact, if we take into consideration 

 the small included crystals of aegirite and riebeckite in the feldspar 

 and consider them as original crystallizations from the magma, we are 

 forced to conclude that all of the minerals have been growing during 

 the entire period of the solidification of the rock. This overlapping of 

 the crystallization periods appears to be a characteristic of rocks 

 of this type.^ If, on the other hand, we disregard the small in- 

 cluded aegirite, and riebeckites, it seems clear that the feld- 

 spar and riebeckite were first to individualize from the magma. If 

 aegirite be an original mineral, and there appears no adequate reason 

 to think otherwise, its period of growth must have begun well back in 

 that of the original riebeckite and extended beyond it. In portions of 

 the pegmatites and granite where aegirite predominates, or where 

 riebeckite is wanting, the aegirite appears to some extent contempo- 

 raneous with the feldspar but the greater pert is distinctly later. 

 Throughout the granite and the massive parts of the pegmatites, the 

 aegirite is closely associated with the development of the quartz as is 

 also the zircon. It has been generally noted in riebeckite rocks that 

 zircon has crystallized from first to last. In the present case, the 

 mineral does occur in the feldspar and riebeckite, but the bulk of it is 

 characteristically associated with the quartz, indicating a preference to 

 a late development. 



Murgoci, in discussing the origin of riebeckite, in the article pre- 

 viously alluded to, assigns a role of the utmost importance to mineral- 

 izers and variations of pressure as factors controlling the formation of 

 riebeckite and aegirite in rocks of the Quincy type. He advances the 

 idea that the question as to whether riebeckite or aegirite develops in 

 a magma of this kind, depends entirely on variations in the pressure 

 and the amount of mineralizers " (Fl, Na, Ti, Zr, etc.)" obtaining, or 

 present at any given point in the crystallizing magma. Mineralizers 

 are, doubtless, a leading characteristic of such magmas, and were cer- 

 tainly present in the Quincy-rock magma, but the evidence thus far 

 available does not seem sufficient to guarantee to these mineralizers 

 so critical a rule. In fact the evidence gleaned from the present in- 

 vestigation, while it does not in any way deny to mineralizers an im- 

 portant role in the crystallization of the rock, seems to argue against 

 the hypothesis referred to. The two minerals are not dimorphous, for 

 they differ sharply in chemical composition. The aegirite for example, 

 contains, 31.86 per cent of Fe203 and 0.87 per cent FeO ; the riebeckite 



' Murgoci, Am. Jour. Sci., 33, 133 (1905). 



