Vol. 65.] SUCCESSION AROUND PLYNLIMON AND PONT EKWYD. 495 



pyrite, but that mineral often shows a tendency to develop crystal- 

 line faces whereby the detailed structure of the graptolite becomes 

 obscured, although its form is retained. The following is a list from 

 this locality : — Monograptus cf. argenteus, M. argutus (c), M. 

 communis var., M. cf. concinnus, M. gregarius, M. leptotheca (v.c), 

 M. cf. millipedia, M. cf. mirus, M. cf. triangidatus, Mesograptus 

 magnus, Ghjptograptus sinuatus, Glimacograptus hughesi (c), and 

 CI. scalaris var. normalis (? 'J . 



The pale greenish mudstones which succeed are somewhat 

 monotonous in character and have yielded no fossils, but they 

 are relieved by thin bands of dark shale from which graptolites 

 can be obtained. The only serious gap in the section is where 

 the footpath crosses the stream: a synclinal axis must lie here- 

 abouts, for when the beds are next seen they dip westwards and 

 continue so to do for about 25 yards, when they turn over suddenly 

 in a sharp anticline which is well-exposed in the southern bank 

 of the stream. At this point is a shaly band (F. 25), weathering 

 to a rusty brown with a bronze lustre ; it yielded a great variety 

 of graptolites, among which the following have been identified : — 

 Monograptus convolutus, M. liarpago (v.c), M. limatulus (v.c), 

 M. lobiferus, 31. regularis, Orthograptus bdlulus, Ghjptograptus 

 tamariscus, Glimacograptus hughesi, and CI. scalaris. 



Monograptus harpago is extremely abundant, occurring in matted 

 aggregates on some of the surfaces. Exactly 25 feet above it- 

 is another shale band (F. 26) which contains abundance of grapto- 

 lites, the following forms having been identified from it : — 

 Bastrites approximatus, B. capillaris, B. phleoides, Monograptus 

 ansulosus, M. convolutus var. (c), M. decipiens, M. tenuis (1 '), 1 

 M. harpago, M. limatulus, M. lobiferus (v.c), M. cf. nudus, 

 M. regularis, Orthograptus insectiformis, Petalograptus palmeus y 

 Cephalograptus cometa, Glimacograptus hughesi, and Gl. scalaris (?). 



This band can be followed along its strike up the steep bank on 

 the south of the stream, and opportunity is thus afforded of seeing 

 the same band both in its weathered and in its unweathered state. 

 The difference in the appearance of the graptolites under these 

 different conditions is sometimes striking. A few feet above the 

 prolific baud last described is a thick band of shale, weathering 

 deeply to a creamy white with ferruginous stains, especially along 

 joints (F. 27). It splits into thin flags, or into exceedingly brittle 

 papery laminae. The graptolites are represented by impressions 

 of the flattened polyparies, but their outlines are quite distinct. 



1 According to Miss Gr. L. Elles, the graptolite described in 1843 by 

 Portlock in the ' Report on the Geology of Co. Londonderry, &c.' (p. 319 & 

 pi. xix, figs. 7 a-1 b) as Graptolithus tenuis is identical with the one described in 

 1868 by H. A. Nicholson (Quart. Journ. Geol. Soc. vol. xxiv, p. 539 & pi. xx, 



"!. 12-15) as Graptolites discretus. Portlock's name has therefore priority 

 over Nicholson's. The form which Prof. Charles Lapworth and others have 

 referred to Portlock's M. tenuis comes from a much lower horizon, and agrees 

 with the one described in this paper as Monograptus atavus, sp. nov. (p. 531). 



Q. J.G. S. No. 260. 2 m 



