374 KEPorvT — 1888. 



■well recognisable grains of quartz and felspar. So at Bodafon (106, 107) 

 and Ty Croes (85) coarser and finer rocks are associated. In the Eastern 

 district we find such at Wugan (201), and at Pen-y-parc, Beaumaris (193), 

 ■where the original fragments "were recognised by Professor Bonney. The 

 most characteristic occurrence, however, of these coarser rocks is in the 

 South Stack Series. In this group there are numerous unexamined fine- 

 grained rocks, but almost all those which have tempted examination have 

 turned out to be gritty in charactei", the original quartz grains being 

 closely packed and very uniform in size. Of these may be cited as 

 examples the rock from the bottom of the gorge at Gogarth (59), and a 

 flaggy bed at the lighthouse steps (60) ; also one from Porth-dafarch (61), 

 and one from Porth-y-crug, near Roscolyn (63). Even when there are 

 larger fragments imbedded the gronndmass of these rocks is of a coarser 

 character, as in the quartzites of Porth-y-gwalch (64) and of Roscolyn (65), 

 and in the neighbouring darker rocks {^(J). The rocks, therefore, of this 

 series are as well marked by their microscopic structure as regards their 

 original composition, as they are by their bedded character as seen on the 

 large scale. 



Bodes containing Larger Fragments in a Finer Groundmass. — These fall 

 into several categories — those in which the principal part is groundmass 

 of minute elements, ■with only a few of the characteristic fragments scat- 

 tered here and there ; those in -which the fragments constitute the prin - 

 cipal part, and the groundmass is reduced gradually to a minimum ; and 

 those in which the groundm.ass is also of coarse elements. The first of 

 these groups, in which the fragments are comparatively rare, approximate 

 very closely to the finer-grained rocks, especially when both have under- 

 gone crystallisation. Such are well seen in the eastern end of the Northern 

 district — as at Pengorphwyfsa (222), Point .zElianus (224), and Llaneilian 

 (223), whei'c there are but a few angular pieces of quartz and felspar, 

 possibly of volcanic origin. As we pass west, towards Pen-bryn-yr 

 Eglwys, a focus of eruption, these fragments become more numerous, as 

 at Llechog Ucha (221), in the slates between Llanrhydrns and Canilyn 

 (213), at Mynydd Mechell (211), till they become most numerous near 

 Llanfechell (219), forming a grit largely composed of fragments of quartz 

 and felspar, and containing also pulled-out pieces of a minutely crystalline 

 substance. This is the rock of which Prof. Bonney recorded that it con- 

 tained almost certainly fragments derived from the older series. It does 

 not, however, seem certain to the wi'iter that these pieces ai'e anything 

 more than a peculiar vein substance, such as occurs in several rocks, and in 

 some cases they may be decayed and reconstituted felspar. In the Western 

 district, besides the laminated rocks, to be described further on, there are 

 several of this group associated with the finer-grained rocks, as at Caer- 

 deon (24), beyond Porth Dryw (13), and in the tuffs of Church Bay 

 (28). In the other districts only one of this group has been met with — • 

 viz., in the northern part of the Eastern district, at Coedmawr, near 

 Llanfaes (200). It is suggested that these fragments for the most part 

 are volcanic ejectamenta, contributed to the finer deposits, in which there 

 is in genei'al a complete absence of stratification either on the large or 

 the microscopic scale. 



The group with abundant fragments in a less abundant groundmass has 

 not been observed in the Northern district, which was probably further 

 removed from the sources of eruption, but in the other districts such rocks 

 are numerous and important. In the Western district two of great interest 



