180 MR. E. E. COWPEB REED ON THE GEOLOGr OF [May 1895, 



all shapes and sizes, so that the weathered summits and slopes of 

 the bare rocky prominences are covered with a confused assemblage 

 of tumbled blocks. Such is the case at Llanwnda, and on the 

 ' Cams ' of the Llanllawer mountains. 



Fine spheroidal jointing, such as has been described by Prof. 

 Bonney in his important paper in 1876, 1 is well shown near Aber 

 Clawdd Pridd, and less perfectly near Porth Lleuog and in the 

 rotten decomposed rock on the western slopes of Fishguard Harbour. 



Two excellent examples of columnar jointing have come under 

 my notice ; one occurs near the jetty on the east side of Fishguard 

 Harbour, and the other on Pen Anglas. In the former case the 

 hexagonal columns are fairly regular in shape, and nearly horizontal 

 in position. 



Brecciation in place, such as has been described in the Dimetian 

 of St. David's by Prof. Hughes, 2 is exhibited in the diabase of the 

 little harbour of Pare y Morfa, east of Castle Point. 



(2) The Crystalline Intrusive Rocks and their Mineralogical 

 Composition. 



There is a considerable number of varieties of intrusive rocks in 

 this area ; basalts, dolerites, andesitic dolerites, with tachylytic and 

 variolitic modifications, as well as diabases and gabbros, have been 

 recognized. Most of the crystalline rocks are of a dark greenish 

 colour, and fine in grain. Some are extremely coarse-grained, and 

 the large augite- and felspar-crystals are very conspicuous, as in the 

 Llanwnda gabbro. In some there are patches or veins — contempo- 

 raneous, segregatory, or subsequent — of a finer grain, as in the 

 Y Garn diabase, near Pont Iago. Others are uniformly extremely 

 fine-grained, and are black, heavy rocks, as, for example, a basalt 

 near Penrhiw. Others are vesicular, as near Fishguard Harbour 

 and on Cam Hendy. Others, again, look almost white from the 

 abundance of the felspathic element, as in the case of some andesitic 

 dolerites on Cam Ingle. 



Others, owing to secondary processes, have become violet-grey, 

 compact rocks of a tough siliceous character, with subconchoidal 

 fracture and with spherical amygdules of quartz which have the 

 appearance of white pebbles on weathered surfaces. Near Trwyn 

 Llwyd, on Pen Caer, such rocks occur. 



The principal primary mineral constituents of these rocks are 

 felspar, pyroxene, and iron oxides ; but the relative proportions of 

 these are subject to great variation. Thus we get some rocks con- 

 sisting mainly of felspar (trachytic differentiation), as in the case of 

 those from Garn Fechan, Carn Ingle, and Ehos Hes Cwm, near 

 Dinas. Others are composed almost entirely of large crystalline 

 plates of augite (pyroxenitic differentiation), and of these we find 

 an example in the Llanwnda gabbro. Others again contain iron 

 oxides in great abundance, as in the basalt of Carn Hendy, in which 



1 Quart. Journ. Geol. Soc. vol. xxxii. (1876) p. 140. 2 Geol.Mag. 1883, p. 306. 



