208 T. M'KENNY HUaHES ON THE SILUKIAN 



111 this cliff I picked them up, and traced them S.W. for some 

 distance in the bed of the stream that runs down Nantcaweddu. 

 Here they begin to show a very variable character. In one place 

 just south of the moorland boundary-wall, they contain quartz 

 pebbles from the size of a pea to that of a hen's egg. In another 

 they run into fine sandstones. I here obtained one fossil from the 

 coarser part, which, though not well preserved, can with great pro- 

 bability be referred to Favosites alveolaris. At about one mile 

 south of Corwen a slight fold throws the Grit outcrop about | 

 mile to the east, beyond which I followed it west of the shooting- 

 box known as Liberty Hall, and lost it south of Moel Ferna. Along 

 this line it is frequently very thin and is generally of a finer texture 

 than in Nantcaweddu, being very often a grey-and-white sandstone 

 with wavy lines of bedding and subordinate patches and lines of 

 slate. 



To return to Corwen. The Grit runs along the cliff south of 

 the rectory to Nant Llechog, where it may be seen rolling towards 

 the north, the stream following the face of the beds along some of 

 the folds for a considerable distance. It is then lost under the drift 

 and talus north of Penyglog. Nearly south of Bonwmuchaf the 

 Grit, which is rather coarse, weathering yellow or white, contains 

 what look like fragments of cleaved slate ; but this is not clear, as 

 elsewhere it certainly contains small pans and lenticular patches of 

 mud ; and these, when pinched up in a bed of different lithological 

 characters, such as sandstone or grit, might often appear like included 

 fragments, and have cleavage produced in them alone. In such 

 cases there is generalty a kind of uniformity in the direction of the 

 cleavage-planes ; but when the cleavage-planes of the included 

 pieces of slate lie in all directions, the probability is that they are 

 fragments of a previously cleaved rock. 



Along this line of section the Grits are frequently seen overlying 

 Bala Shale, in which, immediately below the Grit, I found, in one 

 place, Orthis Actonice. In the Grit itself here I have found only 

 some undeterminable fragments of Orthis and Petraia. 



In Nant Llechog the Grit is represented by a white saccharoid 

 sandstone, sometimes ripple-marked and false-bedded, sometimes 

 with black lines and bands of slate in it, at others quite homo- 

 geneous. In several places along this line of outcrop I noticed in 

 the Bala beds a kind of double cleavage, to be referred to two 

 successive movements causing lateral pressure, not in the same 

 direction, and giving to the rock, when viewed across the broken 

 edges of the cleavage -planes, an elongated lozenge-shaped structure. 

 In the series overlying the Grit there seemed to be but one cleavage. 

 I could not make out the original directions of these several move- 

 ments, as it was clear that there had been many subsequent disturb- 

 ances in that area which had affected both the singly and doubly 

 cleaved rocks — such as, for instance, the great movements which let 

 in the Hafodycalch Mountain-limestone, which, with its associated 

 shale, is not cleaved. This test of double and single cleavage cannot 

 be expected to hold everywhere, as it must frequently happen that 



