358 PEOF. T. G. BONXEY ON ROCK-SPECIMENS 



exactly resemble the quartz in the Dimetian. In each there is the same 

 " dirty " look, produced by numerous inclusions, many of which are 

 irregular in form, more like mineral films, some fluid-cavities with 

 bubbles. The sections contain numerous fragments of felspar, very 

 similar to that in the Dimetian ; in short, they present every ap- 

 pearance of an " arkose " to which granitoid rocks have largely 

 contributed. Six out of the seven slides include well-marked 

 fragments of granitoid rock. A description of the largest in the 

 slide numbered (1) will suffice for most of the others. It is about 

 •2" in diameter, containing some eight crystals or parts of crystals 

 of felspar (rather decomposed) and nearly as many grains of quartz, 

 besides a couple of spots of a yellowish- green mineral, probably re- 

 placing a magnesia-mica. The felspar crystals, which vary much in 

 size, are very like those in the Dimetian ; so is the quartz, and the 

 method of association of the two minerals is the same. In all 

 respects the section of this fragment curiously resembles the slides 

 of " Dimetian " rock. 



There are two other fragments in this slide, the smaller exhibiting 

 a micrographic arrangement of the quartz and felspar {quartz cle 

 corrosion ?) which is common in the Dimetian. Slide (2) has three 

 fragments ; (3) has one ; (4) has two ; (5) has three. I have 

 enumerated only those which are indubitable. Besides this, 2, 3, 

 4, 5, 6, 7, exhibit rolled fragments of a volcanic rock. Some are a 

 brown-stained, rather opaque lava, probably a not very siliceous 

 trachyte or andesite ; others, less common, are varieties of a 

 trachytic rock showing innumerable microliths of felspar (probably 

 plagioclase), with granules of iron oxide, in a base perhaps still 

 glassy ; in one or two occur patches of a green mineral, probabl}^ 

 replacing a mica ; one fragment is a devitrified rhyolite in which 

 the quartz is partially aggregated in nests. One slide (3) has a frag- 

 ment of a rather banded rock, exhibiting the cherty structure 

 characteristic of a " halleflinta." Two or three have quartzite, in two 

 cases schistose ; and one (7) contains a fine-grained quartz-schist, 

 with a structure which reminds me of some of the compressed 

 quartzose schists of the Highlands. One or two also contain bits of 

 an argjllite. 



I perfectly agree with Mr. T. Davies in his identification of the 

 contents of these conglomeratic rocks, and can only say that if a 

 considerable number of the fragments have not been derived from 

 the so-called Dimetian, thej^ have come from a rock which bears 

 the most extraordinary resemblance to it. The latter hypothesis 

 is so improbable that I cannot hesitate to adopt the former. 



Specimens oe Rocks and of Pebbles erom the overlying 

 Conglomerate at Trefgarn. 



The peculiar rocks from Trefgarn and Eoch Castle have already 

 been noticed, as regards their microscopic structure, by Mr. T. 

 Davies and ]i*rof. Blake. Certainly they are not normal felstones. 

 In the field, to the eye, to the touch, under the hammer, they have 



