12 



University of California, 



Vol. i. 



planes but each mineral has a common orientation for distinctly 

 isolated individuals widely scattered through their host. Thus, for 

 a considerable area, all the plagioclase crystals which lie in the plane 

 of the clinopinacoid have a common orientation, extinguishing 

 light simultaneously throughout the area, embracing perhaps a score 

 or more of individual crystals, all sharply idiomorphic. What this 

 orientation is with reference to the crystallographic orientation of 

 the host has not yet been definitely determined. A similar commu- 

 nity of orientation may be observed in the plagioclases lying parallel 

 to the prism faces. 



In the case of quartz, also, not only was this community of 

 orientation established, but the relation of the common orientation 

 to that of the host was ascertained. The law appears to be that the 

 vertical axis of the qtiartz is perpendicular, or very nearly perpendic- 

 ular, to the basal plane of the orthoclase. Thus in Fig. 2 all the 

 quartzes (q) show a uniaxial interference figure, which is only very 

 slightly eccentric to the axis of vision. The eccentricity is constant 

 for all the quartz individuals, and is believed to be due to the slight 

 obliquity of the section to the basal plane of the feldspar. Sections 

 were examined from two other localities, and the same law was 

 found to hold. 



The uniform orientation of quartz in isolated masses intergrown 

 with feldspar is a well-known feature of pegmatitic and granophyric 

 rocks, but the definite relation between the orientation of the quartz 

 and that of the orthoclase, which is here described, has not, so far 

 as the writer is aware, been hitherto observed. The community of 

 orientation of idiomorphic plagioclases inclosed in orthoclase is also 

 apparently a novel and certainly very interesting fact. The orthoclase 

 seems to have exercised a very large control t/pon the distribution and 

 orientation of the separate and distinct individuals of both quartz and 

 feldspar which are inclosed in it. From what has been said of these 

 inclusions, it is clear that we have to deal neither with "Schilleriza- 

 tion" products, nor with the ordinary minute original "interposi- 

 tions," which are common in the feldspathic constituents of igneous 

 rocks, nor with the poikilitic structure of Williams. All the min- 

 erals of the ground-mass occur as inclusions in the phenocrysts, 

 not as minute microlites, but as comparatively large crystals. And 

 the relations of these inclusions to the phenocrysts on the one hand, 



