THE ROCKS OF TDE WASHOE DISTRICT. 61 



of the black border about aug-ite may indicate that this mineral is less iuHu- 

 enced by differences of pressure. The basis of the whole speculation is, 

 however, exceedingly slender. 



Discussion of a zonal piagiociase. — Zoual structurc IS exceedingly common in the 

 feldspars of nearly all the rocks of Washoe, and not infrequently there is a 

 nearly uniform and progressive change in the optical properties from the cen- 

 ter of the crystal towards the periphery without demarkation into zones Of 

 course such a feldspar may be regarded as consisting of an indefinite number 

 of zones, but while ordinary zonal crystals show recurrent layers these do 

 not. 



A remarkable instance of zonal structure occurs in slide 20 from the 

 North Twin Peak. It is illustrated in Fig. 13, Plate III. Thi.s feldspar is 

 probably a labradorite cut on a plane at right angles to the brachypinacoid. 

 The outer edge and the interior kernel extinguish light almost simulta- 

 neously when the cleavage plane makes an angle of about 14° with the 

 })rincipal Nicol section. The intermediate belt, on the contrary, extin- 

 guishes at an angle of only 5°, though in the same direction as the outer 

 and inner portions. The fine stripes are blackest at an angle of about 14°, 

 with an opposite inclination, but they show no zonal structure extinguish- 

 ing light at the same angle throughout their entire length. The persistence 

 of these sti'ipes throughout the crystal seems to prove its crystallographic 

 unity, which is further confirmed by the parallelism of the zonal limits. 

 The section also shows very well the alteration in form of the feldspar 

 during growth, as well as the identity of the zonal inclusions with the 

 groundmass, there being a connection through an opening on one side. 



The variation in the position of the optical axes of different portions of 

 a crystal, the effects of which are seen in zonal structure, must be due to 

 differences in crystallographic orientation, or in tension, or in chemical 

 composition.^ Checks in the growth of a crystal may produce demarka- 

 tions such as are shown in Fig. 1 7, Plate III., and described on p. 60, but 

 so long as composition, tension, and orientation are the same, the position 

 of the optical axes must be constant. In the feldspar under discussion the 

 orientation of the zones cannot be different, and variations in tension would 



' Cf. Min($ralogie Micrograph, by FouquiS & L<Svy, pp. 36 and 130. 



