BY W. H. TWELVETREES AND W. F. PETTERD. 19 



usual forms, and strongly zoned in successive layers. The 

 garnet crystals are habitually intergrown with, and enclose 

 augite. The next most important phenocrysts are those of 

 .a green pleochroic augite, with an extinction angle not 

 exceeding 33°. a light green, c deep green, often encloses 

 apatite. There are occasional large porphyritic crystals of 

 fresh orthoclase and perthite, with zonal tendencies. The 

 holocrystalline groundmass comprises orthoclase laths and 

 allotriomorphic felspar ; sphene in crystals and grains ; 

 some normal biotite ; chlorite in scales as a pseudomorph ; 

 a little analcime and limonite, with purplish iron oxide 

 (manganiferous ?). 



Trachyte. 



Sp. gr. 2-7. 



On Lymington Road, opposite Martin's cottage. 



Macroscupical cTiaracters. 



A bold exposure on the west bank of the road of a 

 pearl-grey granitoid rock resembling a fine-grained syenite, 

 but essentially trachytic in nature. The groundmass is of 

 even granular texture, with a feiv larger crystals of glassy 

 ■felspar, with 010 faces and idiomorphic outlines. Felspar 

 makes up the bulk of the rock ; prisms of hornblende 

 .numerous ; augite is present also, but cannot be distin- 

 guished macroscopically from the hornblende. The rock 

 weathers little, but, when affected, the felspars become 

 jellow and opaque. 



Mineral constituents. 

 Sanidine, oligoclase (albite ?), hornblende, augite, sphene, 

 apatite, zircon, magnetite, quartz. 



Microsco2ncal characters. 



Those of a typical trachyte, somewhat near andesite, the 

 main feature being tabular phenocrysts of zoned felspar in 

 a granular f elspathic groundmass. The hornblende pheno- 

 •crysts are numerous enough to be considered as essential 

 constituents. The augite recedes in quantity to an ac- 

 cessory value. It is difficult to locate this rock in any 

 special position in the trachyte group. The forms of 

 felspar are similar to those prevailing in andesitic trachytes, 

 and there is a good deal of oligoclase ; but there is no 

 •development of glass, and the rock is not lava-like in 

 appearance. 



Felspars.— l^omeivic forms prevail. Carlsbad twins 

 with 010 faces ai-e frequent, and zonal structure is 



