Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/285

 immediate Bristol area, and in the West- Somerset and South - Wales districts, convince me that the age of this conglomerate was the coming in or commencement of the Keuper in the area under consideration.

I must not omit also to state that Sir H.De la Beche in 1846 *, and subsequently in his ' Geological Observer'†, discussed this question and clearly defined the geological position and conditions of the dolomitic conglomerate. He has successfully shown that it was intercalated with the associated and succeeding sandstones and marls at many places, and during different periods, in and through the Keuper series, and even " continued up to and may have included the base of the Lias ; " but we must guard against misunderstanding the two conglomerates, their palaeontological contents being totally different.

There are many localities along the north side of the Mendip Hills, on steep or rising ground, where the intermingling of beach-like wedge-shaped masses with finer surrounding and associated sandstones constantly occurs, these finer accumulations not having been (in some places) removed by denudation. Again, we cannot doubt that the superincumbent marls and sandstones cover up or conceal enormous masses of widely spread conglomerate. This is especially the case over the upper coal-shales in the centre of the southern coal-field. The surrounding zone of Pennant and Lower Coal-shales, Millstone Grit, and Carboniferous Limestone also successively have their upturned edges more or less covered by the conglomerate, the so-called "millstone" or " overlie " of the coal-miners, as is proved by almost every coal-pit sunk in the southern basin ; and the contents of the breccia or conglomerate at once determine the source of the pebbles.

The Geological Survey of Great Britain, in their published horizontal sections, have also distinctly shown, over the Mendip area, the recurrence of masses of conglomerate during the deposition of the whole Keuper series, and how not one only, but many beds of breccia, &c, with similar phenomena, and under given conditions, occur along the strike of the palaeozoic land, having been deposited throughout the whole time that the Keuper sandstone and marls were accumulating in a deeper or more open sea.

3. Mode of Occurrence.

To appreciate this fringe or existing remnant of a once widely spread mass of conglomerate nearly surrounding the Gloucestershire and Somersetshire coal-basin, it should be seen and examined in situ.

It has now the appearance of a line of consolidated shingle beaches remaining at their old levels with relation to the palaeozoic rocks on which they rest and from which they were originally constructed. They represent an amount of time so vast, and a mass of old and lost land so great, that the mind almost re-


 * Mem. Geol. Surv. vol. i. 1846.

† Geological Observer, 1853, pp. 476-496.