Geolo(/l/ of Lihidah. 35 F. 



where, aiid a little to the south of which, the Dacite is found lyiiij; 

 on till.' top of the hornfels ridge, as much as 200 yards east of thr 

 fault. It differs, however, from the Dacite west of the fault in thrc 

 respects : — 



i. It forms residual patches on the hornfels. 



ii. It is schistose, and in some cases intensely so. 



iii. It is closely associated with the tiranndiorite-poriihyiy dykes. 

 which occur \rx the hornfels at lower levels than the schistose dacite,. 

 and which, togethei- with the plutonic mass they represent, are 

 responsible for the production of the schists and liornfels. A 

 boulder of the dyke rock was found in the gneissic dacite. But 

 whether this was an included fragment, or the head of a hidden 

 dyke, could not be determined. The presence of other dykes tends 

 to favour the latter view. But, in any case, it is clear that the 

 dykes, though possibly to a small extent simultaneous with the 

 dacite. were in the main later thaii it. 



These dykes are mineralogically similar, but they vary in the- 

 coarseness of the ground mass from a type about three times coarser 

 than the Dacite up to a type where the minerals of the ground 

 mass are nearly as coarse as the phenocrysts. This latter type is 

 developed on a very large scale in a deep valley in north-east corner 

 of block 920b. 



The following is a composite description of the connnonest types 

 found. 



Phenocrysts of plagioclase are abundant, quartz not so common, 

 biotite is abundant, and subordinate orthoclase. Biotite, asso- 

 ciated sometimes with a little muscovite, is often arranged in 

 radiating bunches. (Ground mass consists of quartz and orthoclase 

 mainly, the latter often in square or rhombic sections, with diagonal 

 extinction. l)ut plagioclase, and flakes of biotite are also present. 

 Apatite, zircon, iron oxide, and pyrite (intimately associated with 

 biotite) are present as accessories. 



Relation to the Dacite. — We may safely assume that these dyke 

 rocks and the associated dacite were derived from the same magma 

 basin. In the Dacite. the ground mass contains quartz and felspar 

 in proportions of about 20 per cent, to 13 per cent. (16). In the 

 dykes, this quartz and felspar form crystals whose size increases as 

 the rock becomes more hypaljyssal, until a type is reached where it 

 is hard to distinguish the crystals of the second generation from, 

 those of the first. It, therefore, appears that the only true pheno- 

 crysts in these dykes, and possibly in the deeper-seated plutonic rock 

 also, are the more basic felspars, some of the quartz and biotite, and 



