346 THE JOURNAL OF GEOLOGY. 



elusions of liquids. The opaque ore grains, judging by their ready 

 solubility, belong to magnetite. The groundmass of these rocks 

 consists essentially of pyroxene in well-shaped prisms, lath- 

 shaped feldspar, without twinning lamination or in single twins 

 according to the Carlsbad law and nepheline. The feldspar of 

 the groundmass in all probability is mostly sanidine. Nepheline 

 is abundant and occurs in well-shaped crystals. Small patches 

 of a colorless base occur between the crystalline components. 



The structure of the rocks is hypocrystalline-porphyritic on 

 account of the occurrence of an isotropic base and the repetition of 

 the crystallization of pyroxene, nepheline and feldspar. Although 

 the specimens by their whole habit and structure belong under 

 the head of nepheline-basanite poor in olivine, the presence of 

 sanidine as phenocrysts causes them to form a transition to the 

 group of phonolites. Unfortunately, analyses of these rocks 

 have not yet been made. 



A microscopical examination of the basaltic rock from Pilot 

 Knob, near Austin, Travis County, was made for the purpose of 

 comparison with the rocks from southern Texas just described. 

 The rock was found to be a nepheline-basalt porphyritic with 

 numerous phenocrysts of olivine. The fine grained ground- 

 mass consists essentially of augite-crystals cemented by non- 

 individualized nepheline in very small amount. 



A. OSANN. 



