550 HENRY S. WASHINGTON 



ning of the anorthite crystals, and to a less extent of their mul- 

 tiple lamellae due to albite twinning. Examination by Becke's 

 method shows that there is no difference in refractive index 

 between the substance of the mantle and the orthoclase phen- 

 ocrysts, so that the two are to be distinguished (especially in 

 ordinary light) by the differences of limpidity, while the higher 

 index of the anorthite is strongly marked. The extinction angle 

 of 11° in sections parallel to ^(oio) already mentioned indicates 

 that the mantles are of soda orthoclase. 



The border shows a well-developed micropoikilitic structure, 

 since it contains like the groundmass many small augite and 

 magnetite grains. In consequence of this there is scarcely any 

 difference to be observed in ordinary light between the mantle 

 and the surrounding groundmass, which seems to come quite up 

 to the edges of the feldspar phenocryst. 



Klein observed this feature in several rocks of the region, 

 and Bucca likewise mentions a similar mantle in describing the 

 leucite-phonolite of Bagnorea. 



This mantle must be carefully distinguished from the phen- 

 ocrystic crystal, as it is evidently the product of a distinct and 

 later period of growth, and is almost identical with the holocrys- 

 talline groundmass. The only difference between the two is that 

 under the influence of the pre-existing feldspar phenocryst the 

 orthoclase substance crystallized as a single individual, the orien- 

 tation of this later growth being determined by the nuclear 

 crystal, and small crystals of augite and magnetite being 

 included in a normal way. In the other part of the groundmass, 

 away from the orientating influence of the large feldspar phen- 

 ocryst, the orthoclase substance crystallized at many points as 

 separate small individuals with diverse orientations, forming a 

 normal trachytic groundmass. 



The frequency with which plagioclase is surrounded by 

 orthoclase in parallel position is well known and is noted by 

 Rosenbusch,' though the present case differs somewhat from 

 those mentioned by him. We do not have here the production 



' RosENBUSCH, Mikr. Phys., I, 638. 



