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



usual forms, and strongly zoned in successive layers. The 

 garnet crystals are liabitualh^ 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 jDorphyritic 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. 



Macroscupica I characters. 



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

 pearl-grey granitoid rock resem])ling a fine-grained syenite, 

 but essentially trachytic in nature. The grpundmass is of 

 even granular texture, with a few 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 macroscopicalh^ from the hornblende. The rock 

 weathers little, but, when affected, the felspars become 

 yellow and opaque. 



Mineral constituents. 



Sanidine, oligoclase (albite .^), hornblende, augite, sphene, 

 ;apatite, zircon, magnetite, quartz. 



Mic } 'oscop ica I 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 j)rev^ailing in andesitic trachj'tes, 

 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. — Isometric forms prevail. Carlsbad twins 

 with 010 faces are frequent, and zonal structure is 



