76 MR. H. LAPWORTH ON THE [Feb. I9OO, 



logical change along our datum-line, that a complete alteration took 

 place in the physical condition of the sea-floor between the periods 

 of deposition of the two series. 



(6) Dyffryn Flags. 



Micaceous Flags and Grits (AbJ. — Resuming our section 

 above the grit-band, we follow the Prysg stream as it descends the 

 long dip-slope of the top of the thickly-bedded grit-group. For 

 about 100 yards or so no rock is visible : at the first bend of the 

 stream to the left, however, a thickness of about 30 or 40 feet of 

 the succeeding group is displayed. Grits are still present ; com- 

 mencing at only 2 feet in thickness, they thin out to about 3 inches 

 at the end of the exposure. Intercalated with these, and forming 

 the predominating feature of the group, is a series of thin, platy, 

 gritty flags, and micaceous blue slates, ringing under the hammer, 

 and weathering to a dark bronze tint. This bronze hue often 

 shows an iridescence, which gives to this portion of the group a 

 characteristic appearance. A few specimens of Climacograptus 

 normalis, Lapw. have been extracted from the flags. 



A hundred yards south of the fence over the stream, the grits are 

 represented by a few ^-inch grit-bands only. The greater part 

 of the series consists of hard, micaceous, dark-blue flags and slates. 

 These are often interbanded with thin grits, occasionally showing 

 very fine, close, carbonaceous laminae, and, more rarely, ferro- 

 manganese sandy partings. The most marked feature, however, is 

 the strongly pronounced dark reddish-brown weathering of the 

 exposed surfaces. 



Immediately north of the fence mentioned above, after passing 

 over a few feet of the last subdivision, a decided change in some of 

 the lithological characters is noticeable. The brown tint due to 

 weathering in the lower series has given place to a brilliant red- 

 orange or haematitic colour. The grits are now thicker, averaging 

 3 or 4 inches, but are somewhat sandy in composition. They 

 are dark grey, and generally banded with carbonaceous seams. 

 Other peculiarities of this subdivision are the occurrence of very thin 

 plates of grit, intercalated among the flags, and the occasional 

 presence of patches of white calcite in thin films, showing in certain 

 lights a beautiful pearly lustre. The flags again are much softer, 

 and are closely banded with fine black laminae. From these flags, at 

 a distance of a few j'ards above the foot of the first cliff below the 

 fence, I have obtained Diplograptus acuminatus, Nich., Climaco- 

 graptus parvulvs sp. nov., CI. normalis, and Dictyonema sp. 

 Beyond doubt the most striking characteristic of this subdivision is 

 the pronounced red coloration due to weathering. The colouring- 

 matter is often present in such large quantities as to stain the 

 fingers. 



The total thickness of these micaceous flags and grits, from the 

 top of the Cerig Gwynion Grits to the end of the last sub- 

 division, may be estimated at 180 feet. 



