1870.] ETHERIDGE—BRISTOL DOLOMITIC CONGLOMERATE. 181 
against and forming the gorge of the Nettlebridge River in steep 
mural precipices 50 feet high, and composed of boulders or frag- 
ments tons in weight. 
No valley to the N.K. of Mells is cut down sufficiently deep 
through the overlying marls, Lias, and Oolites to expose the brec- 
cia; but its presence at Wick, north of Bath, on the one hand, 
and at Mells, before named, on the other, clearly proves that the 
removal of the superincumbent secondary strata ranging from the 
New Red to the Great Oolite inclusive would reveal it. 
The northern part of the Bristol coal-basin, or that portion ran- 
eine from Bristol, Holy Trinity, and Wick, on the south, to Cromhall 
and Tortworth, on the north, possesses on its eastern, northern, and 
western sides abundant evidence of this once continuous conglo- 
merate within the basin; but now, owing to the removal of all the 
overlying newer or mesozoic rocks from the centre of the coal-field, 
and the exposure of the Upper Coal-measures and Pennant sandstone 
at Coal-pit Heath, Yate, &c., its presence is only indicated by exten- 
sive and continuous patches resting on the high ridge of land occu- 
pied by Tytherington, Olveston, and Almondsbury, and on to the 
tableland around Henbury, Leigh Downs, and Clifton, all on the 
western side of the north basin. The accompanying diagram (fig. 2) 
exhibits the characteristic condition of the Breccia, and is a carefully 
prepared illustration of the well-lnown mass overhanging the river 
Avon on its right bank, and about 200 feet above the river; this, 
with many others, shows the gradual passage into finer breccia and 
pebbles, and ultimately into the fine-grained sandstones which 
cover up the Clifton Downs west of the Observatory, and the table- 
land to the north. This isolated mass, with other evidence on both 
banks of the Avon, at and above the same level, clearly determines the 
age of the gorge of the Avon to be Post-Liassic in time, and shows that 
the river must have cut its downward or deepening channel through a 
large amount of superincumbent Secondary rocks ; for from the sea- 
level, at Portishead and Pill, &c., up to the height of the reptilian 
conglomerate on Durdham Down, we have a constant succession of 
these beds resting upon the older rocks of the river, and here and 
there clinging to the precipitous faces of Carboniferous Limestone 
or Old Red Sandstone rocks that constitute the gorge of the tortuous 
Avon, and finally spreading themselves under the finer sandstone of 
the Keuper over the great mass of Carboniferous Limestone, &c., 
that constitutes the heights on the western side of the coal-basin, 
300 feet above the sea. 
Crossing the channel we find the same phenomena existing be- 
tween Tidenham near Chepstow, and Pyle near Bridgend, to the 
west. Its northern range is defined and bounded by the southern 
outerop of the Carboniferous Limestone and Coal-measures of the 
South-Welsh coal-field, where, as in Gloucestershire and Somerset- 
shire, it rests in patches upon the older rocks, and, as on the eastern 
side of the Severn, is the source of the calamine, lead, and hydrated 
oxides of iron of the South-Wales area. 
