Cotton. — Geolog// of Signal Hill, Duncdin. 



121 



Phenocrysts. — Large clear crystals of sanidine occur with Carlsbad twinning 

 and cross-parting. In some cases they are elongated, and vary in length 

 from 1 mm. or less to 1 cm. or more. 



Nepheline occurs in sharply idiomorphic short prisms, but only rarely. 

 They are as much as 1 -5 mm. in breadth. 



Groundmass. — Feldspar is very rare. Orthoclase occurs rarely in fine 

 needles, showing Carlsbad twinning. 



Nepheline is the chief constituent. It occurs abundantly in good idio- 

 morphic crystals, in size about 0*06 mm., and also interstitially. It gives 

 the section a honeycombed appearance. 



Associated with the nepheline is abundant mossy aegirine or opgirine- 

 augite of a deep-green colour. It occurs in very small crystals without 

 definite outlines. The extinction - angle measured from the direction of 

 greatest elongation is in many cases low, although it is impossible to 

 determine it accurately. The pleochroism is from green to yellowish-green. 

 At least some of the mineral is, therefore, probably pegirine ; but a?girine- 

 augite may also be present, though to a small extent. 



Magnetite grains are very rare. 



No glass is present, and no cossyrite. 



Order of Crystallisation. — (1) Magnetite ; (2) sanidine and nepheline 

 phenocrysts ; (3) mossy pegirine and the nepheline of the groundmass. 



Chetnical Composition. — The analysis of a tinguaite from Alno, Sweden,* 

 is quoted for comparison : — 



Numerous flows of this rock occur, making up the chief basalt mass. 

 All are very similar. Only one, which differs somewhat in texture, is briefly 

 noticed separately as basalt No. 2. 



Macroscopic Characters. — The rock is black and basaltic-looking, with 

 numerous crystals of olivine and feldspar. No true columnar structure 

 is to be seen, but some weathered boulders have the appearance of square 

 prisms. Spheroidal weathering is general, and everywhere the solid rock 

 is covered by a mantle of clay with spheroidal cores. 



Microscopic Characters. — The structure is holocrystalline and porphyritic. 

 Large phenocrysts of feldspar, augite, and olivine occur in a groundmass 



* Rosenbusch, Elemente. p. 215, 1898. 



