Vol. 58. | DISTURBANCES IN BRAYSDOWN COLLIERY. 613 
Mr. McMurtrie has taken the view that the increase in overlap 
and uplift of the Radstock Fault in the seams of coal, from the 
Great Vein down to the Bull Vein, is due to the commencement of 
this fault before the deposition of the Upper seams, and to a con- 
tinuance of this movement during their deposition’: and in a recent 
paper he says :— 
‘ A natural inference would be that the lateral movement (whatever its cause) 
pee commenced before the deposition of the Upper Veins, and that it must 
have been continued afterwards-—either as a gradual movement, or in successive 
periods, which would fully account for the more limited overlap in the upper 
strata.’ 
The facts observed at Braysdown Colliery show that this could not 
have been the case; for, had this been so, there should be a marked 
difference in vertical thickness between the corresponding strata 
lying between the same seams on the north and on the south side 
of the faults. At Braysdown Colliery there is practically the same 
thickness of strata between corresponding seams on both sides of the 
fault. And, so far as I have heard, this is the case with other 
overlap-faults of the Somerset Coalfield; except, perhaps, where the 
seams lie in close proximity and are much contorted. 
It appears to me, therefore, that we must come to the conclusion 
that these overthrusts in the Somerset Coalfield did not take 
place until after all the seams of coal in the Radstock Series had 
been deposited ; but, from some cause or other, the rate of overthrust 
was continuously lessening above the Bull Vein, as we find that 
the amount of overlap diminishes in each seam above this one. 
Dead Ground. 
In the Radstock Series, areas are frequently met with, in some of 
the seams, where the coal is entirely replaced by soft black shales. 
Such areas are known locally as ‘dead ground.’ So far as I am 
aware, no attempt has been made to account for these areas of 
‘dead ground’ in this coalfield, the prevailing idea being that 
they are ‘ wash-outs,’ that is to say, that the coal has been washed 
away by currents of water previous to the deposition, of the roof 
above it. From observation in the mine I am led to believe that 
the areas of dead ground are not wash-outs, but that they have 
been produced by the differential movement of the strata, which also 
originated the overthrusts before mentioned. _ 
If they were wash-outs, we might expect the space washed out 
to be occupied by the same strata as those which form the roof; but 
in almost every instance the place of the removed coal is occupied 
by the same material as that which constitutes the floor, namely, soft 
black shales. The points of contact also of the coal and dead 
ground do not suggest wash-outs (see fig. 2, p.614). The areas are 
usually irregular in shape, but when surveyed are generally quite 
1 See Greenweli & McMurtrie’s ‘ Radstock Portion of the Somersetshire 
Coalfield’ 1864, p. 18; also J. McMurtrie, ‘ Geological Features of the Somerset 
& Bristol Coalfields’ Trans, Inst. Min. Eng. vol. xx (1901) p. 331. 
