ON THE OLDER EOCKS OF ANGLESEY. 383 



pend also on the greater or less homogeneity of the material. Thus, in 

 the Eastern district, where the rocks are more thoroughly quartzose, the 

 elements are larger ; bat in the Central district, where felspar and quartz 

 are about equally abundant, both are of smaller size. In this last case 

 they commonly show laminar orientation, and have a peculiar aspect, like 

 a mass of boiled sago, especially well seen by the use of the paraboloid. 

 Such are the ' grey gneisses' of Gwalchmai (69), Bodwrog (72), Pen-y- 

 carnisioo- (73), north of LlangwUog (78), and south of Holland Arms 

 (154). Why the metamorphism should be greater in one place than 

 another is not easy to say. Certainly in one case there is a neighbouring 

 mass of o-ranite, and in the other a large number of intrusiye masses, yet 

 the phenomena have no relation to contact metamorphism. But the 

 presence of these masses, on the one hand, indicates the proximity of 

 heated conditions ; and on the other, when pressure was brought to bear 

 upon the rocks, they might act as buttresses. There is no special evi- 

 dence to connect any of these crystallisations with particular intensities 

 of pressure. 



With regard to the orientation of these rocks, we have every kind 

 exemplified amongst them. The quincuncial orientation is, as before 

 stated, best shown when the elements of mica are small. It is beautifully 

 seen in the rock at Porth-y-fawch (75), in which the mica crystals are 

 exceedingly sharp and clean, and two or three of them often stand end to 

 end. At Llangwyfen (77) maxima and minima of mica occur in alternate 

 bands, the former being probably a later product, developed in parallel 

 cracks. The rock at Penlon (142) shows a sago structure in its other 

 elements, and at Mynydd Mechell (211) there are numerous derivative 

 fragments. 



The linear orientation is especially characteristic of the Eastern dis- 

 trict. It is well seen to the east of Gaerwen (156), where magnificent 

 crystals of mica run in irregular lines, or crowd in bands, the other 

 elements being quite unorientated. In this and two others in the neigh- 

 bourhood garnets are found, probably as the final result of ciystallisation. 

 The rock in Llangaffo cutting (150) is remarkable for the straightness of 

 its mica, and that at Berw Ycha (152), which is full of quartz fragments, 

 shows that, in spite of these, crystallisation of the matrix can be carried to 

 its furthest extent, and linear orientation still be carried out, in remark- 

 able contrast to the quartzite of Holyhead. This is also the character 

 of the orientation in the two rocks of the Central district at Gwalchmai 

 Tm'npike (71) and Porth-y-ly-wod (74), already referred to as containing 

 large mica crystals. Both of these, however, are more dusty rocks, the 

 former from the presence of actual dust, the latter containing granular 

 epidote. The laminar orientation of the sago-like grey gneisses of the 

 Central district has already been mentioned. In three of them — viz., at 

 Pen-y-carnisiog (73), Gwalchmai (69), and Bodwrog (72) the orientation 

 is assisted by quincuncial mica ; but in that north of LlangwJlog (78) 

 there is very little mica at all. 



Elemental orientation, which would seem at first sight to require more 

 pressure, and therefore more metamorphism, does not appear to charac- 

 terise anywhere the grey gneisses or mica schists, but rather to occur in 

 localities removed from these, where the rocks are generally surrounded 

 by less altered types. The only exceptions to this are at Bodowyr (146) 

 and Hafodty (144), in the Eastern district, and these appear to owe their 

 character to a pressure acting subsequently to their first metamorphism. 



