OF RHYMNEY AND PEN-Y-LAN, CARDIFF. 487 
List of Fossils from Upper Ludlow, Rhymney River Section. 
Rhynchonella nucula, Sow. Pterinea Sowerbyi, M‘Coy. 
Obolus Davidsoni, var. transversus, Mytilus mytilimeris, Conrad, 
Salt. Murchisonia Lloydii, Sow. 
Rhynchonella borealis, Schl. Huomphalus sculptus, Sow. 
Discina, sp. funatus, Sow. 
Atrypa reticularis, Linn. Phacops caudatus, Brinn. 
Orthis elegantula, Dalm. Beyrichia gibba, Salz. 
filosa, Sow. Cyathophyllum pseudoceratites, M‘Coy. 
Spirifer elevatus, Dalm. Monticulipora pulchella, 1.-Hdw. 
plicatellus, var. radiatus. Alveolites, sp. 
Chonetes striatella, Dalm. Syringopora bifurcata, Lonsd. 
The beds just mentioned form the beginning of a low bank, 
which continues to afford small exposures for some distance north- 
wards. The beds so exposed are all of the Ludlow type, exceed- 
ingly like those of the Usk district, greyish and brownish mudstones, 
with numerous layers of iron-stained fossils, among which Chonetes 
striatella, Orthis lunata, and Strophomena euglypha are the most 
abundant. They continue for 74 feet vertically above the first beds 
of the bank, and then give place to genuine Old Red sandstone and 
shales; the change takes place very rapidly, Ludlow beds, full of 
fossils, lying on one side of a small ditch, and deep-red or purplish 
flagey micaceous sandstones on the other. Just above the highest 
Ludlow and below the Old Red Sandstone a brownish grit, speckled 
with grey felspathic particles, occurs. It abounds in fragments of 
fish-bones, which, however, with the single exception of some 
spines of Onchus tenwistriatus, are too obscure to be even generically 
identifiable. I have never been able to find the bed itself, only the 
fragments which lie upon its outcrop; but Mr. Storrie by digging 
beneath these has discovered the bed in situ, just about the place 
where I had concluded it must occur. 
The general succession of the Silurian beds in the Cardiff area 
may be summarized in the accompanying general section (fig. 4, 
p- 488), in which the thicknesses are taken from the Rhymney river 
exposure. 
The total thickness of the Silurian beds exposed is thus about 
950 feet; and if the base of the series is Wenlock, as we suppose, 
and not May Hill in age, then the thickness of the Wenlock may be 
taken at 581 feet, and of the Ludlow at 364 feet, the relative thick- 
nesses of the two series being much the same as at Usk, though the 
actual thicknesses are a little greater, as one, indeed, might expect 
from the replacement of calcareous by sedimentary strata. The 
uniform character of the whole series is very striking, alternations 
of sandstone and shales repeating themselves with wearisome 
monotony, and the total thickness of true limestone not exceeding 
3 or 4 feet at the most. The prevalence of sandstone beds, often 
exhibiting ripple-markings and oblique lamination, and the numerous 
fragments of plant-remains, all point to a shallow sea not far from 
land. Even the limestone beds, such as they are, point to the same 
conclusion ; for the interstices between the shells of which they are 
chiefly composed are not always filled up with limestone, but with 
