part 3] LOWER PAL2EOZOIC OF ARTHOG-DOLGELLEY. 299 



noticeable by reason of its high pyrites content [C 238, PI. XVIII, 

 fig. 6], a feature that is of course not uncommon along igneous 

 contacts. 



The upper part of this intrusion and also the lower part of the 

 neighbouring larger Tyn-y-llwyn intrusion consist of coarser- 

 grained and more acid granophyric rocks, which will be described 

 below under the designation markf ieldite. With them is 

 associated a considerable amount of a peculiar felspathic, sometimes 

 quartz-free type, which may be conveniently referred to as the 

 Tyn-y-llwyn type. This rock consists mainly of a fine-grained 

 ground-mass composed of a mosaic of felspars, idiomorphic to 

 granular, wrapped round by ragged wisps of chlorite [C 242 & 

 C 312]. Among accessories secondary sphene is abundant, in the 

 form of small, irregular, brownish-grey patches ; magnetite and 

 apatite occur as usual ; and there are occasional patches of secondaiy 

 calcite. Distributed irregularly in this fine-grained ground-mass 

 are irregular patches of clear, almost glassy felspars, coarse-grained 

 and free from chlorite [C 226, PL XIX, fig. 5]. Between crossed 

 nicols these clear patches are seen to consist of a number of large 

 crystals interlocking quite irregularly, and reminding one of a 

 jigsaw puzzle. These larger crystals show the peculiar streaky 

 and blotched appearance characteristic of perthitic intergrowths,. 

 the intergrowth being on a very small scale, wherefore twin stria? 

 are always short and constantly interrupted. This felspar seems 

 to be similar to the 



' . . . . variety of albite which has a dappled or speckled character, with short, 

 interrupted twin stria3,' 



found by Dr. Flett in some of the Devon albite-diabases (loc. cit.)~ 

 The rock contains numerous xenoliths of the finer-grained 

 quartz-dolerite, or Waen-Fechan type, which constitutes the lower 

 margin of the intrusion. With increase in the amount of chlorite 

 the Tyn-y-llwyn type passes gradually into the Waen-Fechan type. 

 The occasional absence of free quartz from this Tyn-y-llwyn type 

 is curious, seeing that quartz is a fairly abundant constituent, either 

 as granules or as granophyric intergrowths in rocks such as the 

 quartz-dolerites of Waen-Fechan type which are associated with,. 

 and are even more basic than, the Tyn-y-llwyn type itself. Why 

 quartz should locally fail thus suddenly, remains at present 

 uncertain. 



These Tyn-y-llwyn rocks appear to be similar to the felspathic- 

 diabases from Devon described (with analyses) by Dr. Flett as 

 follows : — 



' . . . . pinkish rock that occurs as segregations in typical albite-diabase» 

 The felspar has the dappled appearance that characterizes the microperthites. 

 Good albite-twinning, however, is often seen in the crystals. About 12 per 

 cent, of the rock must consist of alkali-felspar; the other minerals are chlorite, 

 iron-ores, apatite, and a little quartz not certainly primary. The rock is 

 remarkable for .... its abundant alkalies . . . .' (Op. svpra cit. p. 62.) 



