398 EEPOBT — 1888. 



These rocks are shivered into fragments, and the larger are re-cemented 

 amongst the smaller, presenting a true mortar-structare. It is often 

 difficult to recognise the identity of such rocks with a fresher granite, and 

 it is only in the larger pieces -which have escaped disintegration that the 

 original character can be seen. It is important to examine the structure 

 of the granite where it is so clearly intrusive in its mode of occurrence at 

 Maenfwyn (122). Here it is coarse-grained like the rest, and the felspars 

 are broken and the quartzes microspectral, and there are a few tourmaline 

 crystals, but it is of the same general type as the rest. The surrounding 

 rock, with which it has an irregular boundary, is a dusty schist, whose 

 small elements have partially crystallised, and which contains large flakes 

 of white mica which have been developed in relation to cracks. 



The remarkable foliated rock at Tafarn-y-botel (123), which has so 

 much the aspect of a Highland gneiss, has a very distinct structure. The 

 felspar and quartz are about equally developed, and neither plays the 

 role of groundmass to the other. The elements are smaller than in the 

 granites, but much larger than in the gneisses of the district. The quartz 

 often occurs as rounded grains, of which many are included in the felspar. 

 There is also a considerable quantity of brown mica, which assists in the 

 foliation. This, however, is principally produced by the presence of a 

 dusty mineral, with well-marked cleavage, which will not extinguish as 

 a whole, and which looks like an altered sphene. The rock is also richer 

 than usual in accessory minei'als, zircon occurring in the quartz, and 

 apatite in the felspars. 



The quartzless group of these rocks is more local, being found only in 

 the neighbourhood of Llecheyn-farwy. It is difficult to give a name to 

 these rocks, of which the bulk is felspar and a large proportion plagioclase, 

 while the other minerals have undergone a change into chlorite or calcite. 

 Such is the rock near Llecheyn-farwy church (124). Its large close-fitting 

 felspars are thoroughly speckled with small crj'stals of calcite or sericite ; 

 there are a few scattered areas which may have been hornblende or mica, 

 but are now mostly changed to chlorite, and there are zircon, apatite, 

 and leucoxene as accessories. There is no observable alteration due to 

 pressure. 



Another rock of this type, at Ynys Dodyn (118), requires great caution 

 At first it appears to be in a fresher condition, but a closer examination 

 indicates that it has undergone a cataclastic modification, with a re- 

 formation of its crystals. Plagioclase is still the principal ingredient, 

 and, in certain patches, it is much speckled and contains in the intervals 

 between its ci'ystals the remains of mica now altered to chlorite. These 

 patches are separated by, and often isolated in, continuous areas of fresher 

 plagioclase in which there is no mica, but epidote fills the intervals 

 between adjacent crystals. Thus the speckled portions appear to repre- 

 sent the original rock, which has broken up, and the fresher felspar has 

 crystallised between and around the fragments. It is remarkable that, as 

 in the case with the mica schists, the later pressure has produced spectral 

 polarisation in the fresh elements, while the original fragments are left 

 unaffected. There are other cataclastic forms of this type in the neigh- 

 bourhood in which recrystallisation has not gone so far, which need not 

 be further specially noticed. 



Associated with the granites of this district are several quartz felsites, 

 of which two have been examined from Tyn-y-PwU (119) and Pen-y- 

 carnisiog (114). It is by no means certain from their stratigraphical 



