OF RHYMNEY AND PEN-Y-LAN, CARDIFF, 493 
sion, and very different from that presented by a truly organic cal- 
careous rock. 
After the foregoing observations, which are to be regarded more 
or less as a digression, we may return to our determination of the 
thickness of the Old Red Sandstone in the South-Welsh area, with 
the object of drawing one or two further inferences from it. 
As we have already shown, the Old Red Sandstone thins out from 
a total thickness of 10,000 or 15,000 to one of 4000 feet in passing 
from the north to the south side of the South-Welsh coalfield ; and 
from this it would appear that the existence of 13,000 feet of so- 
called Devonian strata in Devonshire is not by any means so 
remarkable a fact as it has usually been considered ; for if the Old 
Red Sandstone diminish, as it does, to the extent of one third of its 
total thickness in crossing the Welsh coalfield, a distance of 19 or 
20 miles, certainly the Devonian strata may diminish to a like 
extent in crossing a like distance, viz. from the North Foreland over 
the Bristol Channel to the nearest exposed base-line of the Old Red 
(that is, near Cardiff). 
In the next place, the simultaneous thinning-out southwards of 
the Old Red Sandstone and Silurian of South Wales, and the accom- 
panying change of character in the sediments of the latter formation, 
alike point to the existence somewhere in the neighbourhood of the 
Bristol Channel of land or a submerged barrier, which the thickness 
and proximity of the Devonshire strata forbid us to regard “as 
haying had a great extension towards the south; in other words, 
the land which our observations indicate is no other than the 
famous barrier which Mr. Etheridge was led long ago to invent 
in order to divide the Old Red waters from those of the Devonian 
ocean. 
Again, accepting the existence of such a barrier as our observa- 
tions plainly point to, then we may perhaps be able to explain by 
its means the intercalation in Devonshire of sandstone beds having 
Old Red characters, such as the Foreland sandstone, Hangman grit, 
and Pickwell Down sandstone, with other sediments, both limestone 
and slates, having true marine and Devonian characters. 
Let us suppose first that the depression of our barrier beneath the 
sea proceeded, as a rule, with such extreme slowness as to 
maintain generally a separation between the Welsh and Devonian 
areas ; each of these areas may then be subject to independent pecu- 
liarities of condition contemporaneously, and red sandstones be laid 
down to the north while marine limestones are forming the south. 
Next let us suppose that the rate of subsidence of the barrier 
became at intervals increased, so that the Devonshire and Welsh 
areas became one; it would now be possible for similar conditions to 
exist throughout the conjoined area; Old Red Sandstone conditions 
or Devonian conditions might either of them prevail over the whole 
of the Devonshire and Welsh region: @ priort we could not say 
which would so prevail, or whether both would exist together ; but 
looking to the extension of sandstones with Old Red characters into 
Deyonshire, and the absence of Devonian strata in Wales, we may 
