OF RHYMNEY AND PEN-Y-LAN, CARDIFF. 489 
= 
Distribution of Land and Sea. 
After the deposition of the Cambrian * (Sedgwick) a wide-spread 
upheaval seems to have taken place over the Welsh area, producing 
continental land on the west and north ; this is indicated partly by 
the difference in character of the Silurians in the east and west 
Welsh districts, and partly by the overlap of the Silurian at Llan- 
deilo-fawr onto the Cambrian, of the Old Red Sandstone onto the 
Cambrian at Llanddarog, and of the Carboniferous onto the Cam- 
brian at Selbeck. 
The thicknesses of the Silurians in the east and west districts 
respectively are given in the following table. 
Sedimentary. Calcareous. 
Eastern districts. feet. feet. 
Nien clad eerie etn... 6 on ane ae 3600 350 
NGI CUMS! enters eee ces ss ae aees 3650 300 
NVCOIR ODES tee ieee ayes a eh ea 2540 250 
IND asy pci Os eae tee te. ty ae 2740 550 
UUESTES saaces Seon aeons eR rea ame Sz 1100°@) ~~ 270 
Western districts. 
Survey section crossing Clun Forest .. 11,000 0? 
BION GT : AEA page Benen tec tele steed eRe ae 5250 10 
I DIBNMC ONES A eRe eect ccc crete nena ae 6400 0? 
NSAI CUCKS) a epee noe he ee ee 3460 0? 
Rhymney (Wenlock and belles only). 950 4 
From this it would appear that the source of sediment was north 
and west, and that the area of greatest subsidence also lay in that 
direction. This is true of the Silurians; and I believe it holds good 
for the Old Red Sandstone as well. What, then, are the relations of 
the Devonshire to the Welsh area? One has hitherto supposed that 
the Silurians were absent in Devonshire, the oldest rocks being the 
unconformably overlain Cambrians of Start Point; and certainly 
this would accord very well with the approach to an area of mini- 
mum subsidence in a southerly direction, which a study of the Welsh 
Silurians seems to indicate. On the other hand we have that 
immense development of Devonian strata in the southern district, 
which, to my mind, is most feasibly explained by Mr. Hull’s hypo- 
thesis ; and if so, I think, considering the general thinning-out of the 
Siluro-Carboniferous strata of South Wales from N.W. to S.E., that 
we are driven to conclude that the Devonshire and Cornish area is 
a totally distinct one from the Welsh, and was formerly separated 
from it, if not by a narrow ridge of elevated ground, at least by a 
more or less stationary sea-floor, which would practically be quite 
as efficient a barrier. 
Relations of the Cardiff Silurians to the Old Red Sandstone. 
Wherever a junction is exposed, the Old Red Sandstone of this 
district succeeds the Silurian with the most perfect conformity. 
Passage-beds appear to be wanting. 
* Wherever the term Cambrian oceurs in this paper, it is used in a Sedg- 
wickian sense, 
