110 



PETROLOGY OF THE ALKALINE ROCKS 



pilotaxitic flow arrangement in the microphenocrysts ; sempatic ; mediophyric to 

 miniphyric. The tuff portion has typical aschen-structur. (Plate III, fig. 4.) 



Included fragment. This contains large first-generation phenocrysts of acid 

 labradorite, and smaller corroded phenocrysts of titaniferous augite, olivine, apatite, 

 and magnetite. All these range in size from phaneric to aphanitic. Smaller 

 patches of isotropic material possess the characteristic form and inclusions of leucite. 

 The base contains felspar laths with the properties of anorthoclase forming one- third 

 of its bulk, and the remainder consists of glass. The tuff portion of the slide contains 

 the same kinds of mineral. 



The rock is therefore a leucitic trachydolerite. 



J. 56 (1124). This is a somewhat similar rock to the inclusion in the pre- 

 ceding. It is, however, megaporphyritic hiatal, dopatic, hyalopilitic. The acid 

 labradorite phenocrysts of this rock are frequently surrounded by a deposition of 

 magnetite belonging to the effusive period of crystallisation. 



J. 41 (1950). Trachydolerite, Inaccessible Island. — This rock is similar. The felspar 

 phenocrysts are somewhat more acid, being oligoclase andesine, and a little biotite is 

 present. 



