474 PKOE. T. G. BOKNEY ON A SECTION 



chytic lava and lapilli, associated with finer materials, grannies of 

 iron-oxide and some viridite, most of this matrix being doubtless 

 decomposed volcanic dust or felspathic mud. The quartzes vary 

 from subangular to rather rounded, the smaller grains generally 

 being the more angular. With a low power they appear rather 

 clear; but with a J-inch objective a considerable number of cavities 

 are detected, generally very small and often associated, mostly con- 

 taining very minute bubbles, which usually move freely. In a 

 grain in the fourth of the above-named specimens the fluid appears 

 to be stained a reddish- brown colour. The felspar is usually a little 

 more rounded externally. It is rather decomposed, but I recognize 

 in some cases orthoclase, and think this predominates, as I obtain 

 but rarely indications of the characteristic twinning of plagioclase. 

 Of the fragments of igneous rock there are several varieties, even in 

 the same slide. Some are slaggy, and in great part completely 

 opaque with opacite ; others are crowded with elongated crystallites 

 of felspar, exactly as in slides from modern trachytic lavas in my 

 collection ; others show a cryptocrystalline structure ; others, brown- 

 banded with ferrite, show a fluidal structure ; some are micro- 

 porphyritic, exhibiting grains of quartz or crystals of felspar. One 

 fragment in the first- named slide, with a rather irregular rounded 

 exterior bordered with black, appears to be " micro- amy gdaloidal," 

 the numerous little vesicles being filled near the exterior with opa- 

 cite, and within with celadonite (or a green serpentinons mineral) 

 and a clear mineral, probably a zeolite. The whole appear to me to 

 indicate the presence of materials truly volcanic — lapilli and frag- 

 ments of trachytic lava. Some, at least, of the materials are water- 

 worn ; and I should suspect that, as a whole, they have been trans- 

 ported to their present position (though the cones from which they 

 must have been derived were probably at no great distance), rather 

 than have been showered down as they now lie from a volcanic orifice. 



The fragments mentioned above, in the second slide, were two, an 

 inch or so in diameter, which lay almost in contact in the first-named 

 grit. Both were a very compact dark rock, but one had weathered 

 to a paler colour. The latter is a rhyolite, with scattered angular 

 grains of quartz and felspar in a matrix crowded with minute gra- 

 nules of ferrite and (apparently) with extremely minute crystallites. 

 There seem to be traces of a glassy base, but this is uncertain. 

 Not so, however, in the other fragment ; this exhibits admirably a 

 fluidal structure, wavy bands, almost opaque with granular ferrite, 

 appearing in a clearer base. As the field, except for some scattered 

 crystallites, remains dark with crossed nicols during a rotation of 

 the stage, and does not sensibly vary its tint when a quartz plate is 

 inserted, I consider that we have here a fragment which has retained 

 its original glassy condition. 



I have compared these slides with a rather numerous collection 



from the district near Bangor which I described in a former paper*. 



The resemblance to some of these is very close. The fine grit macro- 



scopically most resembles that from a pit near Hendrewen ; but 



* Quart. Journ. Geol. Soc. vol. xxxv. p. 309. 



