402 DEPARTMENT OF TEE INTERIOR 



2 GEORGE V., A. 1912 



of their small diameters of 1-5 mm. or less. The ground-mass is composed of 

 a fine-grained aggregate of alkaline feldspars and numerous microlites of 

 augite and hiotite, along with some titanite, abundant accessory titaniferous 

 magnetite, and large prisms of apatite. In several of the thin sections nephe- 

 lite, occurring in small, stout, hexagonal prisms, enters the list of accessories, 

 but it has generally been altered to hydronephelite. Calcite and serpentine 

 are the principal accessory products, the former apparently resulting from the 

 alteration of the rhomb-feldspars, the latter from the alteration of olivine. A 

 variable amount of isotropic matter, almost certainly glass, forms a base within 

 the ground-mass. 



Notwithstanding the development of the secondary products noted, the 

 rock must be regarded as fresh. It is strong and breaks with a sonorous, 

 phonolitic ring. The changes suffered by the olivine and rhomb-feldspars 

 are doubtless due to the action of imprisoned magmatic water, rather than to 

 ordinary weathering. The augite and biotite and the feldspars of the ground- 

 mass are, in the specimens collected, generally quite unaltered — a testimony to 

 the freshness of the rock. 



The specific gravity of this phase varies from 2-647 to 2-751. The higher 

 value applies to a holocrystalline specimen rich in phenocrystic augite. The 

 lower values correspond to specimens with some glass in the ground-mass. 



Rhomb-feldspar. — The phenocrystic feldspar of this phase is more or less 

 opaque and generally of a brownish colour, through the inclusion of small 

 augite, magnetite, and apatite crystals and granules. Twinning is not visible 

 macroscopically. The bounding surfaces of the crystals are never smooth but 

 are affected by irregular shallow bays filled with the material of the ground- 

 mass. Yet the surfaces approximate closely in their positions and relations 

 to the crystal planes characteristic of the phenocrystic feldspar in the Nor- 

 wegian rhomb-porphyry, viz. (110), (110), (201). Both the basal and 

 clinopinacoidal cleavages are well developed. Individual phenocrysts cleaved 

 parallel to the base are roundish or rectangular; those cleaved parallel to (010) 

 have acute-rhombic outlines similar to those of the well-known anorthoclase 

 in the Norwegian porphyry and in the lavas of Kilimandjaro. 



Under the microscope these feldspars are generally seen to be zoned; others 

 are unzoned and then have the same optical properties and chemical composition 

 as the cores of the zoned individuals. The core composes from 50 to 90 per 

 cent or more of each zoned crystal, averaging about 80 per cent of it. 



In its general properties the core is somewhat allied to anorthoclase. The 

 double refraction is relatively low ; the single refraction is somewhat greater 

 than that of orthoclase or that of the outer shell of the zoned individuals. Very 

 often the sections transverse to the zone of cleavages display the fine mesh so 

 characteristic of anorthoclase and due to the simultaneous development of 

 albite and pericline twinning. On cleavage plates parallel to the base the 

 extinction is nearly parallel to tne trace of the pinacoidal cleavage but it may 

 be as much as 1° from strict parallelism. Two sections nearly perpendicular to 



