NOETH-GRUG. — CURVATURES, STRATIFICATION, AND SLATY CLEAVAGE. 353 



one of which occurs on the summit of a low ridge, extending to the north-east from the farm-house 

 of Cwm-clyd, The most nearly perfect of these basins is that at the foot of Cefn-y-garreg, near 

 a spot called Pant-dreinan, where the strata converge to a common centre, presenting the appearance 

 of a large amphitheatre, in which the banks or seats are formed of the successive strata which issue 

 from beneath each other at angles of 25° and 30°. The synclinal axes of these rocky troughs, trend 

 from north-east to south-west, and are therefore parallel to the anticlinal lines of the higher ridges 

 by which they are flanked. In the annexed wood-cut the spectator is looking along the line of 

 strike in one of these synclinal depressions. In the Cefn-y-garreg or north-western boundary of 

 this rugged tract (the ridge seen on the left) the sandstones and grits are again thrown up, first at 

 angles dipping 35° and 40° south-east, and they are flanked by older strata, which dip 70° and 80° 

 north-west. These older, dark-coloured schists with harder beds, occupy the ridge of Cerrig- 

 gwynion, and strike perfectly parallel to that of Cefn-y-garreg, and resuming the true south-easterly 

 dip, underlie all the strata described, and thus graduate upon the north-west into the rocks of the 

 Cambrian System. 



In my third and last visit to this wild spot (July 1835,) I detected fossils in the extreme western 

 flanks of these sandstones of Noeth-griig and Cefn-y-garreg, including a new species of Lituite, 

 Atrypa undata, A. rudis, Productus sericeus, the Ort/m Actonia, and the O. flabellula, &c. (See 

 PL 22.) Some of these fossils are even found in the vertical strata of Cerrig-gwynion and Cefn- 

 llwydlo, and are specifically identical with well-known shells of the Lower Silurian Rocks ; while the 

 beds in which they occur graduate on one side into the Cambrian rocks, and on the other into 

 Lower Silurian Rocks. Occupying, therefore, the base of the latter system, these beds must underlie 

 the Llandeilo flags, which are here represented by a thick zone of black flaglike beds, though I 

 could find no traces of the characteristic trilobites. Hence we may infer that the species of these 

 animals which existed in such great abundance while the lower portion of the Silurian System was 

 accumulating, lived only in certain localities, of which the north-western slopes of the Corndon, 

 Salop, the flanks of the Berwyns, the district of Radnorshire north of Builth, the vale of the Towy 

 from Llangadock to Llandeilo, the western extremity of Caermarthenshire, and parts of Pembroke- 

 shire, are the best examples. 



The principal drainage of the singular tract of Noeth-griig is effected by the union near Glyn-moch 

 of several streamlets, which there traverse in a deep gorge the ridge of Cefn-y-garreg. On the south- 

 west bank, the beds of quartzose sandstone dip 75° north-west, whilst on the opposite bank they are 

 contorted, and in one part appear to have been altered. I could not detect any protruding trap, though 

 it is probable that such rock may lie at no great depth beneath these highly disturbed and dislocated 

 strata. Owing to the slight covering of soil or turf throughout the greater part of this rocky tract 

 around Noeth-griig, all the convolutions and disruptions to which the strata have been subjected are 

 very clearly exposed. 



The district is, further, of high interest in affording instructive examples of the distinctions be- 

 tween the planes of slaty cleavage and the laminae of stratification. The former can be well studied 

 upon the north-western ledges of Noeth-griig, in strongly marked parallel lines, nearly vertical, as 

 represented in the previous wood-cut; and having a north-easterly and south-westerly strike. 

 They appear at first sight to be true lines of stratification. So much indeed do they prevail over 

 other marks of structure, particularly from the projection of the serrated edges of the layers, that 

 it was only when I found these lines cutting through the organic remains, that I was convinced they 

 were the planes of cleavage which had been impressed upon the strata subsequent to their deposition. 

 The parallelism of these lines is strikingly contrasted with the undulating surfaces of the beds in the 



