BY C. A. SUSSMILCH AND H. I. JENSEN. 173 



Handspecimen : j^ellowish-grey rock in which small sanidine 

 phenocrysts can be recognised with the naked eye. 



Microscopic Structure — (1) Texture: hypocrystalline, hiatal }jor- 

 phyritic, with trachytic fabric in the fine-grained base. (2)Com- 

 position : felspar, the dominant constituent, forms over 80 % of 

 the slide. It is always idiomorphic, and occurs chiefly as Carlsbad 

 twins. The crystals have an orthorhorabic aspect, and have the 

 faces w(llO), b(OlO), c(00l) and a;(10l) well developed. Con- 

 sequently the sections obtained are mostly equant (square, five- 

 sided and six-sided), and some rectangular, elongated in the c' 

 direction, or more irregular when cut obliquel}^ Some crystals 

 twinned on the Bn-veno-law, and apparently enclosed by the faces 

 c(OOl), y{201), z{rdO), and 5(010) are also present. Manebach 

 twinning, with c as composition-face, has also been detected. In 

 addition, other complex forms of twinning occur, some approach- 

 ing the albite-, others the pericline-type, but they show as 

 shadowy bars instead of distinct lamellae. Twinning of this 

 kind is probable due to an interlamellation of two varieties of 

 felspar. The extinction-angle measured on the edge cb is from 

 6° to 12° in different crystals. Inclusions consisting of sagenitic 

 rutile, blue amphibole and brown amphibole, occur in the felspar. 

 Cross-cracks parallel to (100) are well developed. From the 

 observed characters it is clear that the felspar has the composi- 

 tion of soda-sanidine, which sometimes becomes microcline-micro- 

 perthite by the development of complex twinning. The next 

 mineral in order of abundance consists of irregular grains of a deep 

 reddish-brown mineral, pleochroic from wine-red to almost black- 

 opaque. The pleochroism is to some extent masked by high 

 absorption. Cleavage is faint. This mineral tends to form a 

 poecilitic intergrowth with felspar, and the blue soda-amphibole 

 clusters round it, the brown mineral merging into the blue by 

 imperceptible gradations. The blue hornblende appears to be 

 riebeckite, and the brown is probably kataphorite. The two 

 together form 7-8 % of the area of the slide. Glass and other 

 isotropic materials (including isotropic chalcedony and opal) form 

 about 10 % of the area. A few allotriomorphic quartz-grains 



