Vol. 65.] SUCCESSION AROUND PLYNLIMON AND FONT ERWYD. 469 



undetermined gasteropod. the only fossils obtained were graptolites 

 of the species Dicellograptus aneeps, 1 Orthograptus truncatus, 

 0. cf. mutabilis, and Climacograptus scalaris var. miserabilis : these 

 are fairly abundant, though not well-preserved. 



East of the farmhouse, the Nant-y-Moch stream cascades over 

 a few feet of hard blue sandy flags, with thin siliceous seams ; they 

 strike in a southerly direction, and probably pass underneath the 

 Dicellograptus Beds. No fossils were obtained from them. Prom 

 the cascade to the Rheidol no rock is exposed in the bed of Nant- 

 y-M6ch ; but, in the centre of the river and along the western bank, 

 blue flags with siliceous and gritty seams occur, which resemble 

 ■closely the beds at the cascade. The graptolitic beds are probably 

 a thin seam in this mass of blue flags, to which I shall refer as 

 the ISTant-y-Moch Flags (see fig. 1, p. 470, and Section No. I 

 on the Map, PL XXIV). 



Prom the banks of the Rheidol the section is continued, with but 

 few and unimportant interruptions, along the base and flank of the 

 conspicuous hill Drosgol, which overlooks the valleys of the Rheidol 

 and of its tributary Afon Camddwr flowing in from the north- 

 west. In ascending the section the blue flags give way gradually 

 to dark-blue mudstones with bands of hard grey grit ; these are 

 followed by a considerable thickness of coarse deposits consisting 

 of alternations of dark-blue mudstones, grey conglomeratic grits, 

 and gritty or pebbly mudstones. The last-named frequently present 

 a curious gnarled and knotted surface on weathering, which is 

 connected with their internal structure, for a fresh fracture shows 

 numerous dark lamina? twisted and contorted in a remarkable 

 manner. It is not certain whether this contortion results from 

 powerful compression of the rocks, or from a kind of concretionary 

 action after their deposition. 



A decidedly conglomeratic appearance is imparted to some of the 

 mudstones by the large well-rounded pebbles of vein-quartz which 

 they contain ; these are sometimes aggregated into definite though 

 impersistent bands, but more usually they are thinly scattered in 

 a haphazard fashion throughout the softer sediment. The origin 

 of these curious pebbly mudstones is a puzzling question, for the 

 conditions which allowed quartz- pebbles as large as hens' eggs to be 

 embedded in a matrix of tine mud are difficult to conceive. 



In the grits and mudstones alike there are numerous scattered 

 cubes of pyrite, often a quarter of an inch or more in diameter ; 

 on exposed surfaces they weather out, leaving a cubical cavity. 



The dip of the beds is in a westerly direction, and the bands of 

 grit and conglomerate can be readily followed by the eye as they 

 ascend the flank of the hill towards the summit. To this mass of 

 coarse deposits I have given the name of Drosgol Grits {Ab) or 

 Drosgol Beds, from the name of the hill where they are so well 

 displayed. 



1 In order not to encumber the text, the names of authors of species are 

 omitted ; they are cited in the general list in Table II, facing p. 530. 



