Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/262

208 In 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 probability be referred to Favosites alveolaris. At about one mile south of Corwen a slight fold throws the Grit outcrop about $1⁄4$ 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 generally 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 Actoniæ. 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 homogeneous. 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 movements, as it was clear that there had been many subsequent disturbances 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