278 PllOF. A. H. COX AND ME. A. K. WELLS ON THE [vol. lxxvi,. 



The slates are rather pyritous, and are dark blue to almost black,, 

 according to the degree of cleavage, the less cleaved material 

 showing, as usual, the darker colour. They weather first rusty, and 

 finally bleach to a pale grey. Ashy material is frequently present,, 

 distributed either sporadically, or in little bands from a fraction, 

 of an inch up to several inches thick. Besides the thin bands we 

 find also thicker ash-bands, definite enough to be traced over 

 considerable distances. One such massive, fine-grained, rhyolitic 

 or china-stone ash can be picked up at intervals from the Arthog 

 Valley to Penrhyn-gwyn and is especially noticeable on either- 

 side of Llynau Crogenen. 



The thickness of the slates is about 800 feet, but is difficult to- 

 estimate exactly. Despite the fact that they produce a marked 

 hollow, exposures are plentiful all along the' outcrop. They are 

 the most uniformly fossiiiferous of the Ordovician slate-bands, and 

 as such proved very useful in elucidating the structure. The 

 characteristic fossils were obtained at numerous localities, as indi- 

 cated on the map (PI. XX) ; the most fossiiiferous locality is the 

 small quarry on the west side of Llyn Crogenen, near its outlet. 

 Here graptolites are extremely abundant, and include Didymo- 

 graptus bijidus, D. stabilis, D. artas, and D. nanus. Trilobites 

 are rare in these beds, just as elsewhere in North Wales. A single- 

 example, probably JEglina caliginosa, was obtained at the quarry- 

 mentioned above. Another, Trinucleus sp., was obtained, in 

 association with Climacograptus and Didymograptus bifidiis, from- 

 slates near Tyddyn Farm, east of the Grwynant. 



(iv) The Cefn Hir Ashes. — The Crogenen Slates are suc- 

 ceeded abruptly by a thick ash-group, consisting, in its lower- 

 portion, mainly of massive, fine-grained, 'andesitic' ashes, and 

 in its upper portion of coarser ashes and massive agglomeratic 

 beds, composed of pebbles of rhyolitic to andesitic material set in a 

 fairly-coarse ashy matrix. The andesitic ashes include a little 

 above their base a band of slate of normal Bijidus type. At the 

 top of the group the massive ashes are succeeded by a small 

 thickness of slaty ashes and slates which, on Cader Idris, have 

 yielded a rather starved-looking Didymogra'ptus bijidus. The- 

 whole group is about 300 feet thick. 



The lowest members of the group are hard fine-grained ashes, which for the- 

 most part represent the finest dust of rather acid 'andesites,' or more strictly ,, 

 of keratophyres, sometimes mixed with argillaceous material now more or 

 less silicified. Some bands are uniformly fine-grained, while others are 

 coarser, and contain numerous small broken felspars and small pellets of 

 ashy material not very different from the matrix in which they are embedded. 

 Although mainly fine-grained the beds are exceedingly massive, and, being- 

 well jointed, they break down into great rectangular blocks. These lowest 

 ashes are 40 to 50 feet thick. 



Above, comes a band of dark slate, rather hard and flaggy, but otherwise 

 similar to the main mass of Bifida* Slates. ' The slates are associated in 

 their upper portion with massive 'andesitic' ashes, composed of interbanded 

 dark-blue and grey material with a very regular lamination. The thickness 

 of the slate-band, with its associated laminated ashes, is about 20 feet. 



