Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/286

 fuses to accept the fact that scarcely one-fourth of the older rock- masses of Somersetshire, Gloucestershire, and Glamorganshire now exist in their original conditions ; and a careful study of the phenomena connected with the deposition and accumulation of the Dolomitic Conglomerate carries with it the conviction that this area, in the West of England, at the close of the coal-measures, on its elevation from profound depths, was subjected to great, and long-continued denudation ; and as the old land slowly rose, its original mass was greatly reduced by the action of the Keuper sea.

Prof. Ramsay, in 1846, in his elaborate memoir upon the "Denudation of South Wales and the adjacent English Counties"*, in discussing the probable presence of the lower members of the New Red Sandstone, suggests that these strata were either " destroyed after deposition or were concealed by overlapping upper beds of New Red, which, still resting in a comparatively undisturbed basin, may conceal the missing members in hidden hollows." This hypothesis is highly probable ; but, up to the present time, it has not received confirmation.

We cannot, however, doubt that this conglomerate with its associated sandstone and marls† is part measure of the waste, and the result of marine and subsequent subaerial denudation under the agency of time ; for whatever inequalities of coast-line or exposed masses existed at the close of the palaeozoic period, it was during the formation of the conglomerates and breccias that they were removed, and the newer masses relatively arranged nearly as we now find them, thus furnishing us with data to calculate approximately the several levels or heights of the older continent above the level of the New Red sea (assuming the latter to have been constant), and thus enable us to find the age of the Dinosaurian conglomerate on Durdham Down near Bristol.

The average thickness of the New Red series over the Bristol area is about 250 feet, less by one-third than the same series in the northern and central part of England.

The lowest member recognized in the Bristol and South-Wales coal-field is the Dolomitic Conglomerate. I may, however, mention that in some localities there are no pebbles ; for at Sully, on the Glamorganshire coast, the rock is perfectly homogeneous, and has the appearance of a dolomitized carboniferous limestone, and in many places the different conditions of these rocks pass so imperceptibly into each other that it is convenient to consider them all under the common appellation of conglomerates.

The average thickness of this conglomerate is about 20 feet, although many sections exhibit mural faces and slopes 40 or 50 feet in height or thickness. Occasionally it becomes so fine-grained and highly charged with the cementing matter, that it assumes the character of a compact dolomite, resembling the yellow magnesian


 * Memoirs of the Geological Survey, vol. i.

† The New Red Sandstone formation in this area is composed of three members : — 1, the Dolomitic Conglomerate; 2, the New Red Sandstone ; 3, the New Red Marls at the summit.