Vol. 51.] THE COUNTRY AROUND FISHGUARD. 175 



brownish area is in some cases marked off from the matrix by con- 

 centric cracks. "With crossed nicols these areas are seen to contain 

 fewer microlites and less felsitic matter than the surrounding 

 groundmass, and the central clear mass is seen to consist of mam- 

 millated chalcedony. This rock, therefore, is a type of nodular 

 felsite, resembling some of those from the Lleyn peninsula. Its 

 specific gravity is 2-66. 



(c) The Pyroclastic Rocks. 



Fine tuffs containing fragments of the cryptocrystalline and 

 rnicrolitic types of felsites are fairly common. They have almost 

 invariably been indurated or re-cemented by a secondary deposit of 

 silica, and are tough, fine-grained, and flinty in appearance, so that 

 in the field it is difficult to distinguish them from the true lava-beds. 

 Clifton Ward states that the highly altered Arenig and Snowdonian 

 ashes are indistinguishable, even under the microscope, from 

 felstones. 



The principal horizons for pyroclastic rocks are at the base and 

 top of each volcanic series ; but they are very locally distributed. 

 Thus at the top of the lava-beds near Newport grey ashes come in, 

 to a thickness of 50 feet or more along the Cilgwyn road, while 

 they are absent or very feebly developed near Cam Pica ; and near 

 Bwlch Mawr, Cam Fron, and Coch y Ceiliog they have very different 

 characters. Nowhere east of Fishguard are they so coarse as 

 at Goodwick. But there are fine tuffs among the coarse breccias 

 between Goodwick and Pen Anglas [27], [7]. 



The tuffs that are found on Castle and Saddle Points, on the sides 

 of Fishguard Harbour, belong to the top of the lowest set of volcanic 

 beds in this district ; they have a dark-grey colour, and white 

 crystals of felspar are scattered about in them, the hand-specimens 

 reminding one of some of the Arenig ' porphyries ' of North Wales 

 [211], [37], [75], [220], [48], and [103]. Some of the sections 

 look so much like lavas that it was only after a study of a large 

 series of slides from these rocks that I was convinced of their clastic 

 origin. On the same strike are the Cwm Bach tuffs [204], T214], 

 and [284] with very similar characters. 



Some interesting ashes composed of fragments of highly vesicular 

 lavas, lapilli, etc., occur on both sides of Fishguard Harbour, but 

 are especially well shown on the west side. They have in the field 

 an entirely different appearance from any others met with in this 

 area, being strongly cleaved and of a greenish-grey, bluish-grey, 

 or purplish colour. Many bear, at the first glance, a great resem- 

 blance to crushed diabases. But under the microscope their true 

 origin is apparent, though they contain a large amount of secondary 

 minerals — calcito, dolomite, etc. Some of the carbonates may be 

 original, for these tuffs accumulated on the sea-floor. One section 

 [98] consists not only of fragments of vesicular lavas, but also of 

 slates, quartz-mosaics, microcrystalline lavas, broken felspar- and 



o2 



