Vel. 58.] OVERTHRUSIS, ETC. IN BRAYSDOWN COLLIERY. 611 
The strata moreover are disturbed and contorted to such an extent, 
that some of the coal-seams are so folded that a vertical shaft 
may pass through the same seam two or three times in succession.’ 
Thus at different collieries, what should be the roof of the seam 
has now become the fioor. 
In the Radstock Series of the Upper Coal-Measures, distant 
nearly 4 miles from the Mendips, there occurs a huge overlap-fault, 
which has not yet been proved in the Middle or Farrington seams, 
as they have not been worked to the same extent. The effect of this 
fault, as proved in the Upper seams, has been to thrust them bodily 
forward one over the other in a remarkable manner, and for a great 
distance. The first seam, or Great Vein, overlaps 140 yards. The 
amount of overlap increases in each successive seam, in descending 
order, until, when the sixth, or Bull Vein, is reached about 80 yards 
below, it amounts to 360 yards.” The vertical displacement or uplift 
also increases in each seam in descending order, being to the north 
and amounting to about 70 yards in the case of the Bull Vein. 
The strike of this overthrust seems to coincide with the long axis of 
the inversion on the north side of the Mendips. This being the case, 
these two disturbances would appear to be in some way connected, 
and to owe their origin to the same lateral pressure. But, whatever 
the origin of this pressure (as to which question there are different 
opinions *), it is probable that the cause of the formation of the 
Radstock overlap-fault also gave rise to a series of other smaller 
and parallel overthrusts on the north side of it. 
Two of these minor overthrusts have been met with at Brays- 
down Colliery, situated about 14 miles north-east of Radstock 
(see fig. 1, p. 610). These have been proved to run parallel one with 
the other and with the larger Radstock overthrust to the south, that 
is, nearly due east and west. The notable fact about these smaller 
overthrusts is that they exhibit the same features as the Radstock one, 
only on a smaller scale. In the case of the overthrust on the north 
side of Braysdown Colliery, the amount of overlap increases from 
about 10 or 12 yards on the first, or Great Vein, to about 80 yards 
on the sixth, or Bull Vein ; the vertical upthrow also increases in 
each seam in descending order, from 12 yards in the Great Vein 
to about 30 yards in the Bull Vein. 
The overthrust on the south side of the pit is somewhat similar 
to that on the north side, but the vertical lifts and overthrusts are 
greater. A peculiar feature here, however, which has been proved 
by actual working in some cases, is, that although the strata seem 
to have been subjected to a severer lateral pressure than in the case 
of the fault on the north side of the pit, the coal-seams have not 
1 Trans. Geol. Soc. ser. 2, vol. 1 (1824) pp. 255-56. 
2 G. C. Greenwell & J. McMurtrie, ‘The Radstock Portion of the Somerset- 
shire Coalfield ’ 1864, p. 17. 
3 See J. McMurtrie, op. cit. and Proc. Bath Nat. Hist. Club, vol. iii (1877) 
_287; H. B. Woodward, Mem. Geol. Surv. ‘Geol. East Somerset & Bristol 
Coalfields’ 1876, pp. 190, ete., and Proc. Geol. Assoc. vol. xi (1890) p. 485: 
w.A.E. Ussher, Proc. Somerset Arch. Soc. vol. xxxvi (1890) p. 88. . 
2 upZ 
