Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/290

 Fig. 2. — Section of Dolomitic Conglomerate in the new road leading from the Hotwells to Clifton and Durdham Downs, showing passage from heavy massive Conglomerate into fine-grained Sandstone.

This breccia largely occupies the country between Barry and Sully Islands and Llandaff, and between the latter place and Llanharry, Coyty, and Pyle, fringing the limestone downs of St. Nicholas, St. Donats, and Nottage, and is, in extent, nearly equal to that of the Mendip range ; indeed, physically, these two masses of old land on either side of the Severn were, and are now, one, being divided only by a deep depression in the Carboniferous Limestone, now occupied by the waters of the Severn. The strike of the submerged limestone is still indicated by three patches at Barry Island, the Wolves' Rock, and the two islands termed the Flat and Steep Holmes, which now evidence that the ridge of high palaeozoic land was once continuous to the peninsula of Gower and Menevia, and onwards, under the St. George's Channel, to the south of Ireland. This axis alone is determinable for 150 miles, over nearly 80 of which the conglomerate occurs more or less, covering much of the once-exposed masses and flanks of the older rocks.

In the Bristol area, however, as noticed by Buckland and Conybeare *, the breccia is chiefly composed of the debris of the rocks on which the conglomerate rests ; and the fragments vary in size from an inch to three or four feet in diameter, many of the larger boulders weighing from one to three tons each.

The conglomerate of the Quantock Hills (of the same age) is constructed entirely of the Devonian slates and limestones of which these hills are composed ; so with the breccias at Milverton, east of Wiveliscombe, and in the vale of Stogumber, between the Quantock Hills and the Exmoor. Porlock and Luckham valleys, although so completely isolated and shut in and to the north of Exmoor, exhibit conglomerates of the same age, and have the same physical aspects. So also at Brandon Hill (Bristol), where pebbles of quartzose millstone grit only form the mass. On the left bank of the Avon, opposite Cook's Folly, the conglomerate conceals the subjacent highly inclined beds of Old Red Sandstone, and is there also entirely derived from the rock upon which it rests.


 * Trans. Geol. Soc. vol. i.