BY H. C. RICHARDS. 151 



shaped, and show broad lamellar twinning as well as 

 simple twinning; in addition to these, somewhat longer 

 crystals of sanidine are seen. The phenocrysts vary from 

 •75 to 1 mm. in length, and the groundmass is made up of 

 an equigranular mass of felspar and secondary quartz, the 

 granules of which are about -1 nmi. in diameter. 



The groundmass is somewhat difficult to determine, 

 but it seems to be constituted of sanidine and albite, through 

 which is distributed abundant quartz which seems to be 

 almost entirely secondary. Limonite is very abundantly 

 distributed all through the rock. The specific gravity is 

 249. Name: Trachtjte. 



Specimen 51. — Localitj^ : Portion 51, parish of Esk. 

 This is an altered rock and somewhat difficult to determine. 

 Apart from the fact that it intrudes the Walloon coal- 

 measures, one has little indication as to its ;ictual age. In 

 the hand-specimen this is a greenish-grey rock, iine-grained 

 and containing occasional phenocrysts of a flesh-coloured 

 felspar. Microscopically, it is seen to be holoerystalline 

 and to be made up almost entirely of felspar. There are 

 occasional stout idiomorphic plagioclase phenocrysts, 1-5 

 mm. in length, set in stout idiomorphic felspars with an 

 average length of -75 mm., while all through the rock are 

 allotriomorphic patches of quartz. 



There are very occasional sub-idiomorphic crystals of 

 augite -6 mm. in diameter which show twinning. All through 

 the rock one finds patches of chlorite, and this has almost 

 certainlj^ been derived from altered augite granules. The 

 plagioclase is very largely albite, as it has an extinction 

 angle of 15° on the (010) face, and a refractive index 

 lower than quartz. Andesine, with a composition AbgAng, 

 occurs frequently. Magnetite and ilmenite are abundant, 

 and occasional crystals of apatite are seen. 



Secondary minerals, such as calcite, chlorite and serpen- 

 tine, occur also ; possibly some of the quartz is secondary. 

 This rock in many respects appears to be a more acid 

 representative of the material which occurs in the same 

 district as augite-andesite, and which is described later. 

 The specific gravity is 2-69. Name : Quartz-trachyte. 



