WIDESPREAD OCCURRENCE OF FAYALITE IN CER- 

 TAIN IGNEOUS ROCKS OF CENTRAL WISCONSIN. 



In central Wisconsin, in the vicinity of Wausau, forming the south- 

 ern portion of the pre-Cambrian district of the northern half of the 

 state, is a wide variety of intrusive igneous rocks. Chief among these 

 intrusives, which constitute perhaps 75 per cent, of the rock forma- 

 tions for many miles around Wausau, is a complex series of holo- 

 crystalline rocks ranging from granite, high in silica, to quartz syenite, 

 nepheline, and sodalite syenites, and related basic, alkali-rich rocks. 

 Associated with this series are an older series of basic rocks and a still 

 older flow of rhyolite, to both of which the various members of the 

 granite-syenite series bear the same structural relations. 



The habitat of the fayalite is in certain phases of the granite- 

 syenite series, and it is not known to be in the other varieties of 

 igneous rock of the region. The mineral fayalite, it will be recalled, 

 is the pure iron olivine, iron chrysolite, having the formula 2FeO, 

 SiOj, and the theoretical composition, ferrous oxide 70.6, and silica 

 29.4 per cent. Considered in its entirety, the series in which the 

 fayalite occurs resembles in many respects, though not in all, other 

 well-known series of nepheline-bearing and related alkali-rich rocks, 

 such as those in Arkansas, in the Christiania region of southern 

 Norway, and in Essex County, Mass. 



The composition of some of the phases of the central Wisconsin 

 alkali-rich rocks is shown in the subjoined table. The analyses were 

 made by Professor W. W. Daniells, professor of chemistry in the 

 University of Wisconsin. 



In the analyses attention is called to the fairly regular increase in 

 amount of alumina and the alkalies as the silica decreases. The 

 iron is abundant and is fairly uniform in amount, with the exception 

 of that in the granite rich in quartz. With respect to the lime and 

 magnesia, these constituents, like the iron, do not differ appreciably 

 in the various phases. Attention is especially called to the uniformly 

 low content of magnesia, which is without doubt one of the causes 

 of the development of the ferrous silicate, fayalite, in the magma. 



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