142 
It has been considered by Geologists that the central 
portion of the Monmouthshire and Dean Forest coal fields have 
not suffered to any great extent, and as far as the exploitation 
has gone in the latter this theory has held good. 
The central portion of the Monmouthshire coal field is 
about 1,500 feet, and that of the Dean Forest 800 feet above 
the level of the Severn. 
Similar Geological causes as those previously referred to 
have separated the Bristol coal field from that of Dean Forest, 
so that the identity of the corresponding seams of coal in these 
districts is a difficult question, and requires further investiga- 
tion in order to arrive at correct deductions. 
The late Sir R. I. Murchison considered that the sandstone 
from the Lower Trenchard coal to the Lower Churchway coal 
were identical with the central sandstones’of Pontypool; and 
also that the beds above the central sandstone represent the 
Upper Shales of the Bristol coal field on the South, and appear 
to be equivalent in position to the various beds above the 
central sandstone of Neath, Swansea, and Llanelly; still there 
is not sufficient evidence collected to enable Geologists to 
decide this question definitely. 
There are two patches of coal measures to the South-West 
of the Forest of Dean, and to the North-East of Chepstow, 
called Tidenham Chase and White Wells, but hitherto a diffi- 
culty has been experienced in determining whether the sand- 
stone occurring there belongs to the Millstone Grit series or to 
the upper portion of the Carboniferous series. No particular 
attempt has been made to compare these small outlying patches 
with the measures of the Forest of Dean with a view to their 
identity. They are, however, marked on the Geological Map 
as Millstone Grit. 
At Howell’s Hill, to the North of the Forest and Bishop’s 
Wood House, and on the West of Hope Mansell, another small 
patch of coal measures occur extending about 1} miles, and of 
an average width of about } of a mile, and some coal has been 
worked from these measures, but they are now abandoned, A 
very peculiar feature connected with this patch of coal 
