Vol. 58.] | OVERTHRUSTS, ETC. IN BRAYSDOWN COLLIERY. 609 
31. OvertHrRvsts and other DisturBANCES in the BRAYspowNn CoLLuiERy ; 
and the Brartne of these PHENomENA upon the Errecrs of 
OvertHRUsT-Favtrs in the Somerser CoaLFIeLp in general. 
By Freprrick Anruony Steart, Esq. (Communicated by 
H. B. Woopwarp, Hsq., F.R.S., F.G.8. Read May 14th, 1902.) 
THE Somerset Coalfield is a remarkable one in many respects. 
Unlike most of the larger coalfields of this country, by far the 
greater part is concealed by Secondary rocks, consisting of Oolites, 
Lias, and Trias, which lie almost horizontally upon the disturbed 
Coal-Measures beneath. Faults in these newer rocks are com- 
paratively few and small. The Coal-Measures, however, present 
a marked contrast in this respect—for this coalfield is in part 
the most disturbed and contorted of those known and worked in 
the United Kingdom. Its area covers about 240 square miles, and it 
is presumably an isolated basin, bounded on all sides by anticlines 
of older rocks. On the north, in the Coal-Measures at Kingswood, 
near Bristol, there is an anticline and overlap-fault, which separates 
the Somerset Coalfield on the south from the Bristol or Gloucester- 
shire Coalfield on the north, on the borders of which the margin of 
older rocks is clearly defined. On the south and south-west the 
great Mendip anticline separates it possibly from a similar coalfield, 
concealed beneath the thicker Secondary rocks on the farther side 
ot the Mendip Hills; and on the east, the anticline which passes 
from the far north near Cromhall, in Gloucestershire, through or 
near Yate, probably continues slightly to the west of Bath and 
thence to Frome on the south. 
The object of this paper is an attempt to describe the overfaults 
and related phenomena in a.single colliery, namely that of Brays- 
down, near Radstock, and to show how the conclusions that may 
be drawn from them account for some of the remarkable disturbances 
which have been locally met with elsewhere. 
In the first instance, it may be useful to recall attention to some 
of the more important phenomena of this nature that have already 
been observed in other parts of the Somerset Coalfield. 
The faults in the Somerset Coal-Measures range from a few 
inches up to 200 yards in downthrow. In some of the collieries they 
are so close together, that it is rare to see a face of coal 200 Yards 
in extent without a fault or some other disturbance. There are 
ordinary faults, step-faults, trough-faults, and—in many collieries— 
overlap-faults. Moreover, the strata in places are inclined at all 
angles. As pointed out long ago by Buckland & Conybeare, in 
the neighbourhood of the Nettlebridge Valley, to the north of the 
Mendips, the mass of the Lower Coal-Measures is entirely in- 
verted; for, instead of rising to the south in conformity with 
the Mendips, the beds dip southward and rise towards the north. 
6G. 8. No, 232. Dag 
