354 GEOLOGY OF TUE EUREKA DISTRICT. 



The phenomenon of zonal variation in the angle of extinction of feldspars 

 Indicates that the chemical composition of the crystals varies from the center out- 

 wards. And as the extinction angle, so far as observed in the feldsi)ars of the ande- 

 site of this district, is usually greater at the center of the crystal than toward the 

 margin, generally passing through a series of distinctly marked zones, which in rare 

 instances have been found to differ by 20°, yet passing frequently by imperceptible 

 gradations from one extreme to the other, it seems likely that during the growth of 

 such feldspars changes have occurred in the chemical composition of the successive 

 shells of enlargement, tending toward greater acidity, which, though often shar])ly 

 defined or interrupted, have sometimes taken place in the most gradual manner possi- 

 ble, a process only conceivable by admitting the correctness of Tschermak's theory. 

 The particular section of twinned feldspar described and illustrated in Fig. 3 has been 

 treated with hot hydrochloric acid. The central portion of both halves was decom- 

 posed and clouded and the zonal structure more strongly emphasized. The marginal 

 zones appeared to resist the attack of the acid completely. This jiroves that the cen- 

 tral portion of the first half, with extinction angles as high as 40° and 44°, is anorthite 

 or bytownite, and that the central portion of the second half is of the same species, 

 but was cut in a i^osition in which the extinction was only 24°. The outer zones are 

 probably labradorite. The difference of their behavior toward liydrochloric acid is 

 more striking than their optical difference. 



The occurrence of anorthite in the volcanic rocks of western America has not 

 been previously noticed, partly because no very thorough investigation of the nature 

 of the plagioclase feldspar in them has been undertaken and also from the fact that 

 all simple crystals showing no strife between crossed nicols, were classed with ortho- 

 tomic feldspar. Thus the simple crystals and Carlsbad twins of sanidine mentioned in 

 Prof. Zirkel's report on the rocks of the 40th Parallel Survey,^ as occurring in such 

 abundance in the " augite-andesite" at Basalt Creek, Washoe, and near Clarks Station 

 and Wadsworth, near the Truckee River, give in the zone perpendicular to the brachy- 

 pinacoid angles of extinction ranging from 0° in a few instances to 40°, thus 33°, 34°, 

 35°, 36°, 38°, 39°, 40°, most of the reading being over 30°, corresponding to those of 

 anorthite. One section cut at right angles to an optic axis showed the plane of the 

 optic axes at an inclination of 43° to the trace of the brachypinacoid. Sinular anor- 

 thite is found in the closely related andesites in the Cortez Range, head of Annies 

 Creek, and on Emigrant Road, Palisade Canyon, and also from the Traverse Mountain, 

 Utah. It occurs in the " augite trachyte,"' from the neighboring Wahweah Range, 

 in the "trachytes"' from Emigrant Road and the south bank of Palisade Canyon, 

 Cortez Range, and in the rock from Jacobs Promontory, Shoshone Range, erroneously 

 determined as rhyolite,^ which is almost identical with the andesite from Richmond 

 Mountain. It wUl thus be seen that anorthite has a very wide geographical distri- 



'F. Zirkel: Micro. Petro., U. S. Expl. 40tU Pai., vol. vi, Washiugtou, 1876. 



