552 PROCEEDINGS OF THE SAINT LOUIS MEETING 



mentary series, and, in turn, lies beneath a later sedimentary series. These 

 igneous rocks may be divided into three series, the oldest being rhyolite, the next 

 diorite gabbro and peridotite, the latest granite, quartz-syenite, nepheline, soda- 

 lite, and aegerite syenites and related rocks. In the last mentioned series fayalite 

 occurs as a persistent, though minor, constituent. It was separated from the rock 

 and analyzed. Phases of the rock and a number of the minerals contained were 

 also analyzed. 



The series in which the fayalite occurs is quite similar to the nspheline-bearing 

 and associated rocks of Arkansas, of southern Norway, and of Essex county, Mas- 

 sachusetts. It differs from them, however, in containing a much smaller amount 

 of magnesia, there being no increase in amount of magnesia as the content of silica 

 in the series decreases. The pyroxenes, amphiboles, and micas present contain 

 very little magnesia, being those rich in lime, iron, and the alkalies. Fluorite is 

 a persistent, though not an abundant, constituent of these rocks. 



The fayalite occurs in the quartz syenite, having a silica content of 61.18 per 

 cent, and to a very small extent in the amphibole-granite, bearing 67.99 per cent 

 of silica. In phases of the quartz syenite it probably constitutes from 1 to 5 per 

 cent of the rock, and was noted in many thin- sections of this rock from widely 

 different parts of the district. In the rocks bearing quartz it is associated with 

 the feldspars, orthoclase, albite, and microperthite, with the calcium-iron amphi- 

 bole, arfvedsonite, with the calcium-iron-pyroxene, hedenbergite, and with iron- 

 mica containing large amounts of potassium. In phases of the non-quartzose and 

 nepheline-bearing rocks of the series the fayalite also contributes a fraction of 1 

 per cent to as much as 5 per cent of the rock. In some of the nepheline-rich rocks 

 the fayalite is the only dark-colored mineral noted in the thin-section. In the 

 latter rocks the fayalite occurs with orthoclase, microperthite, nepheline, sodalite, 

 the soda-amphiboles of the riebeckite and crocidolite type, and the calcium iron 

 pyroxene, hedenbergite, and the potash iron mica, lepidomelane. 



The idea has prevailed, to some extent at least, that fayalite had only an aqueo- 

 igneous origin in rocks rather than ordinary igneous origin. The fayalite in these 

 rocks, however, has the associations and relations of a normal, original constitu- 

 ent. It does not occur in veins, segregations, or cavities, nor does it occur in 

 idiomorphic crystals. It is distributed throughout the rock and assumes in its 

 development such shapes as are due to the mutual interference of surrounding 

 minerals, like the quartz, feldspar, nepheline, and common rock-forming min- 

 erals with which it is associated. 



The alteration of the fayalite is described, and the previously noted occurrences 

 are referred to. 



Remarks on the paper were made by U. S. Grant and 0. C. Farrington. 

 It is published in Journal of Geology, volume xii, pages 551-561. 



The second paper presented was 



FIELD WORK IN THE WISCONSIN LEAD AND ZINC DISTRICT 

 i 



BY U. S. GRANT 



[Abstract] 



During the summer of 1903 the Wisconsin Geological and Natural History Survey 

 conducted some detailed mapping of selected areas in the southwestern portion of 



