360 PEOF. T. G. BONNEY ON EOCK-SPECIMENS 



by Dr. Hicks, of a buff colour, resembles the above in most respects, 

 but there are no cavities, and there is more opacite, vf^hich is ar- 

 ranged in cloudy streaks and patches, suggesting a fluidal structure. 



The slides from Eoch Castle, described by Mr. Davies, exhibit 

 fluidal structure, felspar crystals replaced by chalcedonic quartz, and 

 cavities of various forms, from well-rounded to elongated and irregular 

 in outline, filled by secondary deposits of this mineral. 



Three specimens of the pebbles in the conglomerate at Trefgarn 

 have been examined. 



(15) A pebble of grey ashy-looking rock very like (11). Under 

 the microscope it presents varietal differences, the structure not 

 being quite so certainly that of an ash. The felspar-crystals are 

 less numerous, larger in size, and a greater proportion are plagio- 

 clase. There is also rather more iron oxide than in the rocks num- 

 bered 11 and 12. I think, however, the rock is probably an ash, 

 though it is just possible that flow-brecciation, followed by some 

 crushing, might explain this structure. There is no more difference 

 between this and the last-named rocks than might easily be found 

 in the ejecta of any volcano. 



(16 ) A rather similar rock, probably an ash ; felspar crystals more 

 numerous and ground-mass more completely devitrified than in the 

 last case. 



These rocks are not so well preserved as are those in situ ; they 

 appear also to be a little crushed. Hence it is difficult to be quite 

 certain as to their origin, but I think it is far more probable that 

 they are a somewhat altered volcanic ash. In any case I have no 

 doubt they have been derived from members of the Trefgarn 

 series *. 



(14) A buff-coloured rock, part of a large pebble, which macro- 

 scopically exactly resembles the typical halleflinta of Trefgarn, having 

 the same flinty aspect, &c. The microscopic structure is in all respects 

 identical, except that there are hardly any cavities ; the ground-mass 

 is very slightly coarser and more like that of an ordinary devitrified 

 obsidian. Still it has been to some extent subsequently silicified. 

 When we examine the structure of the matrix with high powers 

 the identity is still more strongly maintained. For instance, this 

 specimen contains a great number of belonites, commonly varying 

 in length from -0005 to -001 inch ; their extinction appears to be at 

 a small angle with the longer edge, and I think it probable that 

 they are hornblende. There are a few specks of ferrite and granules 

 of iron oxide (?) ; these are often grouped irregularly in cloudy 

 patches. The same belonites occur in the two slides from rocks in 

 situ, though in these there are fewer belonites, and there is rather 

 more ferrite. The devitrified structure is singularly alike in both cases. 



I cannot doubt that these pebbles have been derived from the 



^ All who have worked much at volcanic ashes which have undergone con- 

 siderable micromineralogical change, such as those of the Lake District, North 

 Wales, and Charnwood, know that occasionally specimens occur of which it is 

 most difficult to say whether they have been originally glassy lavas or homo- 

 geneous ashes. 



