480 PKOP. T. G. BONNET ON EOCKS IN 



had exposed a similar rock almost exactly at the spot where I had 

 anticipated it would occur. At this place the dip appears to be 

 rather to the N. of E., perhaps E.ISLE., while the dips about Tair- 

 ifynnon are generally about E.S.E., a twist towards the north- 

 ward of which we have other indications near this fault. This 

 breccia, then, retaining its characteristic aspect, has now been traced 

 for above half a mile. 



The peculiar breccia (C), containing elongated fragments of purple 

 slate with smaller bits of felsite, had been traced prior to 1880 from 

 near Tyddn Dreiniog to Cae Seri ; and I had stated that I expected 

 it would occur in the neighbourhood of the Poorhouse. In that year 

 I observed a pit (perhaps 200 yards to the south-east of the building, 

 which had been overlooked at my former visit) in which I found a 

 rock impossible to distinguish from the Cae-Seri rock. Here also 

 the dip (difficult to ascertain) seemed to be about N.E., a direction 

 which is confirmed by other observations in greenish grits and slates 

 in the neighbourhood. 



During a brief visit in 1882 I found traces of a similar rock in 

 the interval between this pit and Cae Seri. This peculiar breccia 

 (C) has now been traced for a distance of full a mile as the crow 

 flies. 



In succession to this comes, I believe, the greenish-grey felsitic 

 grit of the Hendrewen quarry, in which are occasional well-rolled 

 pebbles of similar-coloured felsite. Under the microscope it is 

 seen to consist of fragments (generally rounded) of quartz, felspar, 

 and an acid lava. The last commonly show more or less of a 

 fluidal structure, and, examined with polarized light, are crypto- 

 crystalline ; but two or three are crowded with elongated crystallites 

 of felspar, as may be seen in many trachytes. The materials are 

 rather more decomposed than is usual in the other breccias. There 

 is also one rounded fragment of a quartz grit. The grit described 

 from the quarry by the white cottages west of Minffordd (I think 

 the spot called 1ST ant Gwtherin on the map) has some resemblance 

 to this from Hendrewen, and is probably higher up in the same series. 



So far as I can gather from Professor Hughes's map and paper 

 (vol. xxxv. p. 682), he regards the quarries at Hendrewen and at 

 the back of the Poorhouse as both in Cambrian conglomerate 

 (pp. 690, 692), but the one near Want Gwtherin and" the rock of 

 Cae Seri as part of the Bangor beds. 



The district between B and C is one not easy to unravel. I have 

 zigzagged over it, and found most of the outcrops to be greenish 

 gritty rocks, sometimes pink-spotted ; and near Brynllwyd Earm 

 are conglomerates, one of which contains some fragments of purple 

 slate like that in the Cae-Seri breccia. At this place (e.g. near 

 Careg Hwfa) Professor Hughes again brings in the Cambrian con- 

 glomerate. I cannot, however, say that the rock closely resembles 

 any of the admitted exposures of the latter, though it has something 

 in common with both B and C. Hence I think it more natural to 

 regard these conglomeratic beds as merely local intercalations in the 

 intervening zone. 



