354 GEOLOGY OF THE EUBEKA 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 feldspars 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 sharply 

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

 ble, a process only conceivable by admitting the correctness of Tscherinak'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 proves 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 position in which the extinction was only 24. The outer zones are 

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

 more striking than their optical difference. 



The occurrence of auorthite 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 und also from the fact that 

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

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

 Prof. Zirkel's report on the rocks of the 40th Parallel Survey, 1 as occurring in such 

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

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

 pinacoid angles of extinction ranging from in a few instances to 40, thus 3!?, 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 tin- plane of the 

 optic axes at an inclination of 43 to the trace of the brachypinacoid. Similar auor- 

 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 Wall weah 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, 1 which is almost identical with the andesite from Richmond 

 Mountain. It will thus be seen that auorthite has a very wide geographical distri- 



'F. Zirkel: Micro. Petro., U. S. Expl. 40th Par., vol. vi, Washington, 1876. 



