FAYALITE IN CERTAIN IGNEOUS ROCKS 



559 



it may be stated that the latter differ radically from the alkali rocks of 

 Wisconsin in their much higher content of magnesia. In Brogger's 

 series there is, on the average, five or six times as much magnesia 

 as in these, otherwise similar, Wisconsin rocks. The nephehne 

 syenites and quartz syenites of Arkansas, described by J. Francis 

 Williams,^ are more like these Wisconsin rocks, but the Arkansas 

 rocks, otherwise similar, have much more magnesia than those here 

 described. The nepheline-bearing and associated rocks of Essex 

 County,^ Mass., are generally comparatively high in magnesia and 

 are closely related to gabbro and diorite. 



When the mineralogical composition of these various alkali 

 rocks, from different parts, of the world, is compared, a much more 

 striking dissimilarity is to be noted. The difference, of course, 

 does not lie in the nature of the quartz, feldspar, nepheline, and 

 sodalite, but with the dark-colored silicates; and since the former 

 constitute much the larger portion of the rock, the difference is 

 necessarily accentuated in the less abundant iron sihcates. In these 

 Wisconsin rocks, therefore, the low content of magnesia finds expres- 

 sion in the development of fayalite, and of pyroxenes, amphiboles, 

 and micas comparatively low or free. from magnesia. Whether or 

 not fayalite or other magnesia-free silicate develops in the magma 

 probably depends also, as already stated, upon the oxide condition 

 of the iron. A deficiency of oxygen in the magma, as well as of 

 magnesia, therefore, may be the controlling factors in the development 

 of fayalite; and in those portions of the magma where oxygen was 

 more abundant the more complex ferric-oxide-bearing sihcates, such 

 as aegirite, were formed. 



In comparison with other somewhat similar, alkali-rich rock series, 

 such as the syenite of southern Norway, Arkansas, and Essex County, 

 Mass., it may be stated that fayalite has not been observed in any of 

 them. Furthermore, the pyroxenes present in these rocks are gener- 

 ally the magnesia-rich varieties, such as augite or diopside, which 

 tend to grade to aegirite through augite-segirite and diopside-£egirite. 



The alteration of the fayalite in some instances is quite extensive, 

 the process of alteration consisting merely in the breaking up of this 



1 J. F. Williams, Arkansas Geological Survey, Vol. IT (1891). 



2 H. S. Washington, Journal of Geology, Vol. VII (1899), p. 463. 



