Vol. 58.]  OVERTHRUSIS, ELC. IN BRAYSDOWN COLLIERY. 615 
unlike the course of a stream. Many of these areas contain isolated 
patches of coal in them. 
Dead ground occurs more frequently in the vicinity of faults 
(although it is sometimes met with at long distances from them) ; 
moreover, it often takes a course more or less parallel to faults, and 
especially overthrust-faults. 
Sometimes, in close proximity to dead ground the coal-seam 
presents an abnormal thickness, consisting of layer on layer of coal 
piled one above the other (see fig. 3, p. 614). Such areas of thick 
coal are sometimes extensive, and it is not uncommon in some collieries 
for a seam 2 feet thick suddenly to attain 5 or 6 feet in thickness. 
For instance, at Braysdown Colliery the Great Vein (which is the 
thickest seam in the Radstock Series, having an average thickness 
of 28 inches) has been observed suddenly to reach a thickness 
of 6, 10, and 15 feet, continuing so occasionally for a considerable 
distance and then quite suddenly reverting to its original thickness, 
or more often disappearing entirely, dead ground taking its place. 
Tt is a general rule that thick coal is nearly always preceded 
or followed, sooner or later, by either unusually thin coal or dead 
ground. 
An instructive and remarkable fact which has been observed at 
Braysdown Colliery is, that in the Radstock Series the dead 
ground occurs only, in the majority of instances, in those seams 
of coal which lie on a floor of soft black shale, known locally as 
‘blacks’; whereas where the floor and roof are formed of hard rock 
no dead rock is met with. This is especially noticeable in regard to 
the Bull Vein, which lies almost immediately upon a hard fireclay 
floor; there little dead ground is found. In the place of dead 
ground, however, large areas of very thin coal are common in faulted 
districts in the Somerset Coalfield. 
The case of the Top Little Vein is still more interesting, for this 
seam consists (see fig. 2, p. 614) ofa bed of coal 13 inches thick, lying © 
on a bed of soft black shale about 1 foot thick, with a thin band of coal 
beneath about 4 inches thick, which in its turn lies immediately 
on a hard fireclay floor. This seam, therefore, really consists of two 
beds of coal, the one above lying on soft shale, the one below on 
hard fireclay ; and it is a remarkable fact that large areas of dead 
ground are constantly displacing the ‘ top’ coal-bed ; but throughout 
these entire areas the ‘bottom’ coal-bed lying on the hard floor 
nearly always remains intact. 
Where ‘ thin coal,’ ‘ bad ground,’ or ‘dead ground’ has been met 
with in one seam in this district, the same state of affairs may 
be expected as a rule in the next seam, vertically above or below, 
in the same place or its neighbourhood. There are exceptions 
to this, but it has been proved over and over again to be the general 
rule. We may take one example. Recently the Great Vein 
proved to be very thin and irregular, with frequent intervals of 
dead ground in a certain locality for a long distance. Subsequently 
the Middle Vein, some 27 yards below, was opened out under the 
same places where the Great Vein proved so badly, and almost 
