Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/287

 limestones of the N.E. of England. At Yate Rocks, near Yate and Chipping Sodbury, the yellow conglomerate, similar in all its conditions to that of Clevedon, rests upon the upper beds of the Carboniferous Limestone and Millstone Grit, assuming the most step-like character (c in fig. 1). The edge of the underlying limestone, (a), is denuded to a perfect plane by the agent which deposited the conglomerate.

Fig. 1. — Section at Yate Rocks.

a. Carboniferous Limestone.

b & c. Conglomerate.

This locality exhibits the only remnant of the yellow magnesian beds known in the northern part of the Bristol coal-field. They must, however, have extended along the whole of the eastern side of the basin, from Wick Rocks to Cromhall, where a similar solitary patch attests its northern extension. North of Cromhall, at Tortworth, the conglomerate follows the curve and strike of the Old Red Sandstone and Lower Limestone Shales. I cannot doubt that the yellow beds of Yate Rocks might be exposed in many other places by the removal of the Trias and overlying Lias.

4. Geographical Distribution of, or Area occupied by the Dolomitic Conglomerate.

The palaeozoic rocks of the Bristol coal-field, ranging in time from the Caradoc sandstone to the close of the coal-measures inclusive, are more or less indiscriminately covered by patches of this conglomerate at various heights, from 20 feet to 350 feet above the level of the sea. These rest either upon the Silurian, Devonian, or Carboniferous series, and are the result of that marine denudation which took place during the later Bunter or the commencement of the Keuper deposits.

The Bristol coal-basin occupies an area of about 600 square miles, or a tract of country 30 miles long by 20 wide ; and everywhere within this area, either fringing or capping at intervals the oldest rocks, docs the dolomitic conglomerate occur.

Its northernmost limit is at Tortworth ; and its most southerly extension is outside the limits of the basin, or south of the Mendips, at Worminster and West Compton, the whole equalling a distance of 32 miles, and having masses more or less exposed along the whole western side of the coal-field. The north and south flanks of the Mendips are both continuously fringed and partly covered by beach-like masses varying from 20 to 50 feet in thickness, and in places ascending to the very summit of that range of hills, as at Shipham, North Hill, and Hay don, at Blagdon, East