MONIAN SYSTEM OF EOCJIS. 501 



divided ; but the original arrangement of the rock seems to be that 

 of an unconformity as well as of a fault. This does not make the 

 two rocks belong to two distinct and independent series. The 

 circumstances, of course, had changed ; but the eruption of large 

 masses of rock must be accompanied by such changes. It is not at 

 all to be assumed that there was only one eruption ; the varieties of 

 the " granite " indicate that there were several, and some portions 

 of the dust-rocks may be subsequent to or derived from the earlier 

 masses. 



To the north-east of Llanerchymedd there is a lofty hill called 

 Y Foel. It is used as a trigonometrical station, and largely quarried 

 for road-stone. It consists of Ordovician conglomerate *. On the 

 eastern side it passes into a great beach-breccia, in which there are 

 numerous kinds of rock, of which I made a small collection, and found 

 them to agree, each with each, with samples collected from the slaty 

 series to the east, with the exception of a coarse blue grit, which as yet, 

 I have not found among them. At the commencement, therefore, of 

 the Ordovician era these older rocks must have been already carved 

 into hills and valleys. Between Foel-fach and Tyddyn-bach is seen 

 one of these hills in the form of brecciated grey gneiss, which may 

 be traced some way round the southern slopes of the hill. We thus 

 have a continuation of the older part of the series to the north, and 

 an indication that the Ordovician here forms little more than a skin 

 on the surface. Sir A. Ramsay speculates on the "greenstone" 

 masses to the north of Llandyfrydog being also relics of the more 

 ancient rocks ; but those I have examined are examples of the black, 

 highly crystalline diorites, in some cases picrites, which everywhere 

 jut up amongst the more modern as well as the more ancient rocks, 

 and seem to be of much later date than the latter. 



Summary of the Central District. 



This is divisible into two parts. In the eastern region we have 

 an interrupted upward succession, commencing with a grey gneiss, 

 whose lower part, at least, and its representative further north, in 

 Bodafon mountain, is comparable to the quartz-schists of Holyhead, 

 while its upper part may be more or less the representative of the 

 chloritic schists. These rocks are continued on the other side of a 

 fault by dark, chloritic ashes intermingled with slates and grits 

 and other sedimentary deposits. At either end of this range they 

 develop great agglomerates, and in the southern promontory exhibit 

 the irregularity of volcanic eruptions, mingled with basic lavas. 

 They contain also sporadic limestones, some of which are bedded 

 and possibly fossiliferous and contain nests of jasper. There are 

 also quartz-knobs, which may attain a large size and approximate 

 to stratified deposits. The series may thus be compared with that 

 which lies towards the north in the western district. In the 

 western region we have the largest and most complex volcanic group 



* Described by Prof. Hughes, Quart. Journ. Geol, Soc. vol. xxxviii. 



