part 1] IGNEOUS AND ASSOCIATED ROCKS OF LLA.NWKTYD. 19 



(Z>) The Hardened Sediments. 



The sediments which succeed the rhyolite-breccia are not well 

 exposed. The principal section is in a tiny stream flowing into the 

 Nant Cerdin (marked B on the map, PI. II). Elsewhere the 

 position of these beds is marked by grass-covered slopes. They 

 appear to consist of hardened mudstones, with some gritty or ashy 

 bands. The interest of these mudstones lies in the fact that they 

 have afforded a fossiliferous horizon. At the point marked C on 

 the map (PL II), situated in the higher part of the beds, and only 

 a short distance below the horizon of the spilites, the hard black 

 shales yielded a brachiopod and numerous graptolites. I am greatly 

 indebted to Miss Gr. L. Elles, D.Sc, for the determination of the 

 graptolites. The fauna, though a small one, is quite distinctive: — 



Siphonotreta micvla M'Coy. 

 Dicranograptus rectus Hopkinson. 

 ■Glyptography teretiusculus var. 

 siccatus Elles & Wood. 



Climacograptus schdrenbergi 



Lapworth. 

 Amplexograptus perexcavatus 



Lapworth. 



This association is characteristic of the D icranogrcrptus Shales — 

 particularly of the horizon of the Mydrim Limestone of South 

 Wales, and of the higher part of the Glenkiln Shale (Zone of 

 Dicranograptus rectus) of Southern Scotland. The important 

 point is that this graptolitic horizon occurs below the spilites: 

 hence the latter must be younger. The significance of this fact 

 will be considered in more detail later. 



(c) The Spilites and Spilite-Breccias. 



There are three main exposures of this horizon : — 



(i) In the bed of the River Irfon and on the northern flank of its valley in 



the south, 

 (ii) On each side of the transverse valley of the Nant Cerdin. 

 (iii) In the central ridge of Car Cwm, on the north. 



The spilites vary greatly in their thickness and appearance in 

 the field. In places pillow- structure is well developed ; at other 

 points the rock is quite massive. In colour, they vary from a pale 

 bluish-green to dark greenish-grey, the latter hue being character- 

 istic of the massive spilite of Car Cwm. The rocks are nearly 

 always vesicular, to a greater or less degree. The vesicles are 

 filled, either with some pale mineral (particularly calcite), when 

 ihej are comparatively inconspicuous ; or with a dark-green 

 chlorite having lustrous cleavage-planes, when the rock assumes 

 a conspicuous spotted appearance. The material in the vesicles 

 weathers easily, and the resulting roek has a spongy aspect. In 

 the south, where they form the bold bluff overlooking the river, 

 the spilites are fairly massive in the lower part ; but they have a 

 ■characteristic 'pillowy' structure in the upper part. Here some 

 of the ' pillows ' are of great size, ranging up to 10 or 15 feet in 

 diameter. They generally have an extremely thin, almost glassy 

 trust, followed by a very vesicular band 4 to 10 inches thick, the 

 vesicles becoming fewer towards the centre. The central parts have 



' c2 



