558 



SAMUEL WEIDMAN 



occurring with sodalite and orthoclase. Lacroix has also noted the 

 occurrence of fayahte in the trachyte lava of Mont Dore. In all 

 these instances, with the possible exception of the last mentioned, it 

 will be noted that the fayalite occurs either in cavities, veins, or 

 segregations, and on this account it has been thought that it must 



necessarily be a product of 

 aqueo-igneous origin. 



It may be well, therefore, 

 again to call attention to, and 

 emphasize, the fact that not 

 only does the fayalite occur as 

 a persistent, and in places a 

 fairly abundant, constituent of 

 these various rocks described 

 from central Wisconsin, but 

 that it occurs here as a rock 

 constituent under perfectly 

 normal conditions. 



The presence of fayalite as 

 an essential constituent of a 

 number of phases of this series 

 would seem to indicate that, 

 mineralogically and chemi- 

 cally, these phases are unique. And not only are the fayalite-bearing 

 phases unusual, but also the other phases of the series, since they con- 

 tain no magnesia-bearing minerals, and contain, besides the quartz, 

 feldspar, and feldspathoids, only calcic-iron alumino silicates or alkali- 

 iron alumino silicates, or combinations of them. It is a well-known 

 fact, of course, that all the alkali-rich rocks, such as those associated 

 with nepheline-bearing rocks, contain much less magnesia than lime, 

 and that not only is the proportion of magnesia to lime in these alkali 

 rocks small as compared with the proportion in the usual basic rocks, 

 but that it is also much smaller than occurs in average igneous rocks. 

 In comparison with other somewhat similar and WTll-known 

 alkali-rich rocks, such as the nepheline-bearing and associated rocks . 

 of the Christiania region of southern Norway, described by Brogger,° 



° W. C Broggee, Zeitschrijt fur Krystallographie und Mineralogie., Vol. XVI 

 (1890). 



Fig. 2. — Photomicrograph of quartz- 

 syenite; X6o. The crystals shown are faval- 

 ite, much fractured, on the right, hedenbergite 

 on the left, both surrounded in part by bluish- 

 green arfvedsonite. The black mineral is 

 magnetite, and the colorless crystal is apatite. 



