﻿Vol. 6 1."] ARENra fawr and mobl llyfnant. 613 



Parabolina-Beda. These are dark bluish-grey, and give a sonorous 

 ring when struck : they break with a curious, china-like, conchoidal 

 fracture, and tend to fly before the hammer. They contain a 

 certain amount of iron-pyrites, and coat themselves with yellow 

 rust in the early stages of weathering, although this afterwards 

 disappears and the whole rock appears much darker. 



In the lower parts they are quite unfossiliferous, but about the 

 middle certain bedding-planes yield indifferently-preserved examples 

 of Parabolina spinulosa, Wahl., which becomes abundant in the 

 higher beds. 



The Orthis-lenticularis Band [19] . 



Some 200 feet from the base of the Parabolina-'Beds appears a 

 band of darker and more earthy shale crowded with innumerable 

 shells of Orthis lenticularis, Dalm. This band is only some 4 or 

 5 feet thick, but is readily recognized at intervals all round the 

 Harlech dome from Ogof-ddu near Criccieth to Dolgelly. It is best 

 seen in the old JSTant-y-Derbiniad slate-quarry, immediately north- 

 west of the area mapped, but is also very evident in the western 

 Trinant-stream section, about 100 yards south of the confluence 

 with the eastern tributary. In both these places the Orthis is 

 abundant enough to render the shale quite calcareous. 



The Peltura-Be&a [18], 



Above the Orthis-lenticularis Band begins the famous ' Black 

 Band,' distinguishable from all other beds in the district by the fact 

 that when it is scratched with a hammer the streak produced is 

 quite black. The coming-in of the black coloration is of course 

 gradual, and, with practice, one can tell by the streak exactly with 

 what bed one is dealing. Many have thought that the blackness 

 of the beds is due to graphite or organic matter ; but a study of 

 the way in which the rock rusts in weathering leads me to think 

 that it is rather due to the presence of finely-divided, slightly- 

 decomposing iron-sulphides. When heated alone, the slates become 

 slightly paler, and acids, which will dissolve iron-pyrites but have 

 no action on graphite, completely bleach small pieces of it. 



The sediment composing the Black Band is finer than that of 

 either the beds below or above it, and accordingly has, in places, 

 taken on sufficient cleavage to tempt some persons to open trial- 

 holes for slate. The slates obtained are very soft and silky, but 

 they rust and scale at so early a stage in the weathering that, 

 although easy to work, they have not proved to possess any per- 

 manent value, and all the trials are now abandoned. The best- 

 cleaved material contains abundant distorted fragments of Peltura 

 scarabazoides, Wahl., and fine, large examples of Agnostus trisectus, 

 Salt., are not uncommon. The lower layers near the Or^is-Band 

 yield occasionally Sphcerophihalmus alatus, Bceck, and one or two 

 specimens of Ctenopyge pecten, Salt., have also been found there. 



