COLLECTED BY DR. HICKS IN N.W. PEMBROKESHIRE. 35^ 



a sort of clierty character, which at once attracts the notice of an 

 experienced observer. They differ in these respects from all the 

 old rhy elites of JN'orth Wales, the Wrekin, Charnwood, &c., which I 

 have seen. They have no resemblance to the ordinary intrusive 

 felstones of simOar chemical composition. There is also something 

 distinctly exceptional about their microscopic structure. I have re- 

 peatedly examined during the last few years the slides in my own col- 

 lection and in that of Dr. Hicks, and for a long time I felt the greatest 

 perplexity as to their nature. In some respects they recalled the 

 structure of devitrified rhyolites of a very glassy type (obsidian) ; 

 in others they seemed more like a chert or a much-altered rock 

 of sedimentary origin (hiilleflinta) ; but after seeing Prof. Blake's 

 specimens I felt more satisfied as to their nature, and my view has 

 been borne out by these new examples. 



I proceed to describe the specimens (three in number) obtained 

 from rock in situ beneath the conglomerate : — 



(11) A greenish-grey rock of fragmental aspect, perhaps slightly 

 cleaved, apparently an altered ash. Microscopic examination shows 

 that it is made up wholly of fragments, which are clearly of volcanic 

 origin, — bits of an acid microporphyritic lava full of small felspar 

 crystals, orthoclase and plagioclase (oligoclase ?), — in a base which 

 is now probably wholly devitrified, but was once a glass more or 

 less crowded with opacite and felspar microliths. To enter into 

 minute details would be only to travel over old ground ; the rock 

 may, without hesitation, be named a volcanic ash, which might 

 be either a member of the earlier Palaeozoic series or somewhat 

 older than that. 



(12) A similar rock, but a little more compact and uniform in 

 appearance. The microscopic structure indicates a like origin. 

 One or two fragments are quite opaque from the quantity of 

 opacite, as is often seen in the scoria even of trachytes. Here and 

 there are little nests of an aggregated serpentinous mineral, no 

 doubt secondary replacements of some pyroxenic mineral. This is 

 also an altered volcanic ash. 



I am indebted to Mr. J. J. H. Teall for the use of a third speci- 

 men, a very similar indurated ash. The fragments in all three 

 appear to me rather intermediate in character between an andesite 

 and a sanidine-trachyte, but to be more nearly related to the former. 



(13) A purplish rock with the peculiar flinty aspect of the most 

 typical " halleflintas " of Trefgarn and Eoch Castle. Tnder the 

 microscope it exhibits the characteristic structure of these rocks. 

 With ordinary light the ground-mass is partly clear, partly tinted by 

 extremely minute granules of ferrite, so arranged as to suggest a 

 fluidal structure, though irregular and indistinct. Here and there 

 are either cracks, or small elongated cavities occupied by crystalline 

 quartz and opacite. With crossed nicols the ground-mass exhibits 

 the peculiar speckled aspect which, while bearing some resem- 

 blance to a very minute devitrification-structure, also reminds one 

 of that usual in a chert, and in some halleflintas of sedimentary 

 origin. A specimen from this quarry given to me some years since 



Q.J.G.S. No. 167. 2 c 



