14 ON HAUYNE-TRACHYTE AND ALLIED ROCKS. . 



that of common green hornblende, and not -with that of 

 arfvedsonite, which is a > fi > C But the startling 

 opacity suggests something out of the common, and in 

 some sections the absorption varies to i) > f > a, which 

 characterises the black alkali-iron hornblende of certain 

 phonolitic trachytes and linguaites which Brogger has 

 called cataphorite. It seems to us possible that the horn- 

 blende is of a cataphoritic nature, though its extinction 

 angle is rather low for that species, t : c = 14° to 17°, 

 whereas in cataphorite it varies from 23° to 60°. 



A bright green slightly pleochroic augite occurs in 

 prisms and grains. C : c = 34° or thereabouts. Apatite 

 in grains. The groundmass consists of small sanidine 

 prisms in fluxional arrangement, interspersed with small 

 grains of augite. The whole is rather obscured by de- 

 composition. 



Tertiary haiiyne-trachytes occur in France (Auvergne) ; 

 hatlyne-phonolites in Germany, Portugal, the Canaries, 

 Colorado ; the nosean-phonolite of the Wolf Rock, Corn- 

 wall, is the nearest related rock in Britain. 



Hauyne-Trachyte. 



Found on the crest of the Livingstone Hill, and in the mine tunnel 150 

 feet below. Also in the trench at Mount Mary Mine, west of Lovett. 



2Iacrosco2)ical characters. 



A soft light grey rock, easily recognised by its large 

 tabular orthoclase felspars lying thickly in parallel layers, 

 causing the rock to split more easily in that direction. 

 These felspars are mostly between |" and li" in length, 

 and from ^" to ^" thick, and can be often chipped out 

 from the matrix, making good specimens for the cabinet. 

 They are tabular || 010, and the cleavage parallel to this 

 plane is perfect. The crystals are opaque externally, light 

 yellow, but occasionally the interior is glassy, sanidine- 

 like. Mr. Frank Rutley has aptly described them to us as 

 having a biscuit-like appearance. The miners call this 

 rock " magpie." The only other pronounced macroscopical 

 •element is limonite in hexagonal and other sections after 

 some cubic mineral, probably garnet. The same rock 

 occurs at the Mount Mary Mine, where it is more decom- 

 posed, and contains much epidote, 



Microscojyical characters. 



The orthoclase is often intergrown with a striped felspar. 

 It encloses numerous hexagonal and other sections of 

 haiiyne, replaced by liebenerite (?). The rock is full of 



i 



