ON MANOD AND THE MOELWYNS. 375 



different localities, and that the name " felstone " has covered a 

 considerable variety of rocks. To keep, for the present, to Moelwyn 

 alone, we cannot accept the mapi)in<? of the greater part of the 

 igneous group as felstone ; we find that the rocks deserving that 

 name in the terminology of the field- geologist are very thin in 

 proportion to the fragmentary strata. The whole thickness of 

 igneous rocks can be divided into Lower, Middle, and Upper Agglo- 

 merates, separated from one another by slate beds ; at the base of 

 the lower there is found a thin band of felstone, and in the middle 

 of the upper, one of considerably greater importance. 



The rock we term " felstone " has a fine compact greenish base 

 witliout porphyritic crystals. On weathered surfaces it shows 

 the smooth opaque white crust peculiar to highly silicated 

 rocks. In parts, as in the thick bed below Poel Rydd, there is a 

 well-marked columnar structure, somewhat remarkable for the 

 more rapid weathering of the interior of the columns, producing 

 deep hollows in the top of the columns, and partly hollow centres 

 Avhen seen in longitudinal section. There is no evidence in favour 

 of regarding this rock as intrusive, and, on the other hand, micro- 

 scopic examination shows none of the characteristic structures of a 

 lava. It seems far more probable that we have to deal here with 

 a fine compacted ash ; and this view is very strongly supported by 

 the close similarity apparent between the " felstone " and the matrix 

 of the agglomerates. 



Sir A. Ramsay considered that all the felstones of the district 

 were contemporaneous lavas, and he based his conclusion partly on 

 the presence of a "• flow-structure " in places, but mainly on alteration 

 observed in the lower slates. The former character is not of much 

 value in determining origin, because, as is pointed out in the memoir, 

 appearances indistinguishable from flow-structure are seen in some 

 of the bedded rocks below, and also in the agglomerates. As 

 regards the latter argument, it is probable that the development of 

 spots in the Lower Slate is due to the metamorphic forces that have 

 so altered the rocks below and up to it. On the other hand we 

 have seen with great distinctness the junction of the Upper Felstone 

 with the slate below, and found no evidence of alteration. 



The agglomerates may be divided, as we have said, into Lower, 

 Middle, and Upper beds, especially at the southern end, along the line 

 of Section 1 (see p. 371). Northwards, the Middle bed thins out 

 at the Cwm-Orthin Valley ; while the others form the two clifl-like 

 walls whose detached blocks have covered the mountain-side. The 

 groundmass is compact and light-coloured, closely resembling the 

 felstone in appearance ; on protected surfaces, joint-faces, &c., it may 

 indeed be difficult to distinguish the two rocks, but on weathered 

 faces there can be no doubt as to the elastic character of the agglo- 

 merates. Rounded pebbles and angular fragments of stratified 

 rocks are heaped together in every direction. In some places the 

 embedded rocks are of great size ; one in the Upper bed was found 

 to measure 1 1 feet in length by 4 in width. 



It is interesting to note that there occur in the Upper Agglomerate 



