521 prof. w. ,i. puciii os the geology of the [vol. lxxix, 



in the area. They weather to a dull bronze in most exposures. 

 They are very imperfectly cleaved, and appear to possess a rough 

 double cleavage which causes them to split into lenticular or 

 phacoidal pieces with rather sharp edges. Within a very short 

 distance the}' may become quite arenaceous, and develop definite 

 grit-bands. In other places they contain large isolated masses of 

 grit and gritty mudstone, which are boulder-like in shape, and give 

 to the rock a peculiar gnarled or ' pillowy ' appearance. 



At certain horizons well-marked grit-bands are developed, which 

 are capable of being mapped. The grit-bands may, however, 

 quite suddenly pass into gritty mudstones, which are indistinguish- 

 able from the group as a whole. Further, there is reason to believe 

 that these thick grit-bands do not in all cases maintain a constant 

 stratigraphical position ; and, on account of this, together with 

 their variation in thickness and lateral change, it is doubtful 

 whether any useful purpose would be served by mapping them in 

 detail. In certain places, however, they assist in the elucidation 

 of the geological structure. 



Another somewhat remarkable characteristic of these grit-bands 

 is worthy of mention. Not only may they pass quite suddenly 

 into gritty mudstones, indistinguishable from the group as a whole, 

 but they may within a few feet pass into massive conglomerates. 

 These conglomeratic bands are quite local in their distribution, 

 and are particularly well shown on Mynydd Abercorris, as also on 

 the south side of the Lief enni Valley. They are several feet thick, 

 and consist of well-rounded pebbles in a mudstone or gritty mud- 

 stone matrix. The pebbles may be as much as 5 to inches in 

 diameter, and are composed mainly of vein-quartz, with occasional 

 gritty and quartzitie pebbles. These quartzose conglomerates 

 crop out in irregular patches about the middle of the Garnedd-wen 

 Beds. 



Generally speaking, there are two fairly well-marked grit- 

 horizons : one about the middle of the group (for instance, south of 

 Corns, about 1000 feet below the base of the Valentian) ; and one 

 a little distance above, or immediately above, the Narrow Vein. 



The grits may be examined practically anywhere on the outcrop 

 of the group ; but two typical localities may be cited : — 



(1) South of Corns on Mynydd Braich-goch and Bryn Llwyd, 

 a thick grit-band occurs about the middle of the Garnedd-wen 

 Beds. Owing to its superior hardness, it makes a well-marked 

 feature, and its outcrop may be traced for a considerable distance. 

 It consists of a hard quartzose grit, and immediately south of 

 Corns the outcrop makes a sharp V as it crosses the axis of the 

 Cori'is Anticline. In several places the grit-band is veined with 

 quartz in all directions. 



(2) On Foel Crochan, north of Aberllefenni, the Narrow Vein 

 is overlain by a massive grit-band about 100 feet thick, which 

 makes a conspicuous feature as it crosses over the ridge from the 

 Llefenni Valley into the Ceiswyn Valley. 



One of the most remarkable features of the Garnedd-Aven Beds 



