that long period of denudation which was ushered in with the commencement of the Bunter, and succeeded by the Keuper.
It was during this period that the destruction and removal of the bolder masses of the palaeozoic rocks took place, and this during their depression to great depths in the New-Red ocean, thus producing the accumulated masses of the dolomitic conglomerate, associated sandstones and marls, and added to these the superimposed Lias &c. This depression must have gone on to the extent of some 4000 or 5000 feet*, physical and cosmical changes again (or subsequently) bringing to the surface the old palaeozoic land under newly modified conditions ; and during this slow and gradual emergence the Trias, Lias, and Oolitic rocks were in their turn denuded and swept away, thus giving to the Severn area and valley the general physical features and aspects now presented to us.
We thus have to do with phenomena belonging to two distinct and extended epochs of time, — the first being the removal and remodelling of the prior-existing older and newer palaeozoic rocks, through the advent of the Mesozoic period, and by the agency of the Permian and Triassic seas (which cut back and denuded the coastlines then exposed to their influence), and the deposition in the deeper regions, and along the strike of the shores, of the spoils of the older continent.
The second epoch was that later, even almost modern period of geological time when by the reelevation or reemergence of the accumulated secondary rock-masses, and their subsequent removal, the old surfaces, if not still deeper ones, became again exposed and remodelled, assuming fresh geographical outlines dependent upon the amount of oscillation the land then underwent in relation to the stability of the ocean-level. Thus many of the fissures and faults in the palaeozoic rocks have been twice exposed and influenced, and through great periods of time ; and no one can witness the remnants of some doubtful Permian and Triassic rocks which rest upon the higher lands occupied by the Carboniferous Limestone and Millstone Grit of the district, or examine the mineral veins, fissures, faults, and joints, and their mode of occurrence, without, being strongly impressed, if not convinced, that the conditions thus briefly noticed are those which bear out the hypothesis I have enunciated to account for the accumulation of the ores of iron and zinc, sulphate of strontia, and occasionally manganese. An example of this remodelling of the very same conglomerate is now exhibited along the eastern shores of the Severn between Portishead and Clevedon, where the waters of the estuary are now daily, constantly, and effectually removing its barrier of magnesian breccia, as well as the underlying mountain-limestone, Pennant, and Old Red Sandstone, and again reconstructing the whole into a quaternary conglomerate or modern breccia, but with a different cementing constituent.
I am inclined to believe that this later period of denudation must have occurred during the lengthened era of the Miocene or later
- The Bristol coal-field is between 5000 and 6000 feet deep.
