part 2] ITS STEUCTrEE and eock-succession. 139 



Frondderw Ash.^ 



There is no hard-and-fast line to be drawn between the sandy 

 mudstones and the ash, for wisps of ash at first show themselves 

 in the sandy nandstones, then a thin band or two of definitely ashy 

 material is commonly found, and finally a bed of massive ash is 

 seen, having a maximum thickness of 12 feet; towards the top it 

 behaves much in the same way, mudstone wisps making their 

 appearance, and the whole passing into an ashy mudstone in which 

 the ashy material gradually diminishes until it has completely dis- 

 appeared. The massive portion of this rock makes a conspicuous 

 feature in the landscape wherever it occurs, and at close quarters 

 is easily recognizable by its rough weathered surface, upon which 

 the fragments are sometimes very conspicuous. It seems to be 

 typically developed in the more northerly parts of the area, 

 becoming thinner and less distinctive towards the south, where it 

 is associated with a local development of limestone, and is split 

 into two thin bands with mudstone between them. When last 

 seen (Cefn Gwj'^n) it is merely an ashy mudstone quickly becoming 

 indistinguishable from the surrounding sandy mudstones, which at 

 this locality have yielded some interesting Ophiurids (^Protaster 

 Salter i). 



This ash has been most extensively quarried throughout the 

 district for walls and farm-buildings, so much so, that nearly all 

 the exposures on the north-west side of the lake have been worked 

 all along their strike, and the rock is only now visible at the very 

 base of a small cliff of sandy mudstone, and in many cases is 

 largely concealed by rubble. One of the most conspicuous of these 

 outcrops lies north of Frondderw. Under the microscope this ash 

 is seen to be of a very felspathic nature, being made up mainly of 

 fragments of a coarsely vesicular felspathic lava, some of which 

 are devitrified, fragments of a coarseh^ crystalline rock suggestive 

 of an intrusion, and a few pieces of an oolitic limestone. It is 

 definitely less siliceous than the Pont-y-Ceunant Ash. 



Allt-Ddu Mudstones. 



The sandy mudstone series above the Frondderw Ash quickly 

 takes on a different character from those below ; while the beds 

 immediately above the ash are very similar in their general 

 character to the sandy mudstones below, and contain certain bands 

 with concretions of a practically identical nature, softer beds come 

 in with increasing rapidity. The beds quickly lose their pro- 

 nounced sandiness, beds of this nature being confined to narrow 

 partings only a few inches thick; the colour becomes dark bluish- 

 grey instead of pale blue-grey, and the rocks are often iron-stained 

 in irregular blebs : the whole series is intensely cleaved, so much 



^ I am indebted to Dr. E.. H. Eastall for kindly confirming- my examination 

 of the slices of this and the Pont-y-Ceunant Ash. 



