Correspondence. 523 
but in Paleotherium there is a gradual diminution of size from p 4 
to p 2, and a sudden one inp 1. In Tapirus, p 3, p2, and p 1 of 
the upper jaw progressively depart from the true molar type, as well 
as diminish in size. RicuHarD OWEN. 
PERMIAN STRATA IN THE VALE OF CLWYD. 
To the Editor of the GroLtocicaL MAGAZINE. 
Dear Sir,—The letter from Mr. Davies in your last number 
compels me to again trespass on your space, with a little further 
explanation of the evidence upon which I premised the existence 
of Permian strata in the Vale of Clwyd, and the erosion of the 
Carboniferous rocks before their deposition. 
I fully agree with most of the generalisations expressed in the 
early part of Mr. Davies’s letters, and I can assure him that the 
points he mentions had not escaped me in weighing the evidence 
bearing on the age of the strata at Pentre Celyn. 
When strata become either entirely ‘thinned out,’ or reduced in 
thickness by causes connected with their deposition, the reduction 
in their bulk rarely takes place abruptly, but generally by a some- 
what regular rate of diminution. For instance, if a particular for- ~ 
mation diminishes to half its thickness in a given distance, say 20 
miles, it will only have lost one fourth of its thickness in half the 
distance. 
The Lower Carboniferous rocks in North Wales and the west of 
England are remarkably consistent in this respect over a very large 
area; and there is perhaps no class of evidence which more conclu- 
sively proves that the various separate masses and outliers of Car- 
boniferous Limestone and Millstone Grit once formed an unbroken 
and connected deposit, than the regularity with which their thick- 
ness diminishes both from the NW. and SW. towards the middle of 
England. In the north-west of Wales the Carboniferous Limestone 
attains a thickness of at least 800 feet. At Llanymynech, on the 
borders of Shropshire, its thickness is reduced to about half; and 
following the same rate of diminution in a south-easterly direction, 
the Limestone is only 30 or 40 feet thick round the flanks of the 
Wrekin, and becomes entirely lost under the Shropshire Coal-field. 
The same regularity of diminution is observable from the SW., 
where by regular steps it becomes reduced in bulk from the great 
mass underlying the Bristol and South Wales Coal-fields, through 
Farlow in the SW. of Shropshire (where it is considerably dimin- 
ished) until it thins out to nothing in Shropshire and Staffordshire. 
The Mountain Limestone, overlain by a great mass of Millstone 
Grit, occurs but a few miles to the east of Pentre Celyn. Exposed 
as bold escarpments, and about two miles to the north, a small out- 
lier of the Millstone Grit crops up in the vale. 
At Pentre Celyn no Millstone Grit occurs, and the Limestone is 
unusually thin. Now, as the absence of the Millstone Grit at this 
point, on the theory of ‘thinning out, is entirely inconsistent with 
its general uniformity in North Wales, or any rate of diminution at 
