184 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 12, 
that long period of denudation which was ushered in with the com- 
mencement of the Bunter, and succeeded by the Keuper. 
It was during this period that the destruction and removal of the 
bolder masses of the paleeozoic rocks took place, and this during 
their depression to great depths in the New-Red ocean, thus pro- 
ducing the accumulated masses of the dolomitic conglomerate, asso- 
ciated sandstones and marls, and added to these the superimposed 
Lias &e. 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 palzozoic land under 
newly modified conditions ; and during this slow and gradual emer- 
gence the Trias, Lias, and Oolitic rocks were in their tum denuded 
and swept away, thug giving to the Severn area and valley the 
gencral 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 re- 
modelling of the prior-existing older and newer paleozoic rocks, 
through the advent of the Mesozoic period, and by the agency of the 
Permian and Triassic seas (which cut back and denuded the coast- 
lines then exposed to their infiuence), 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 rela- 
tion to the stability of the ocean-level. Thus many of the fissures 
and faults in the paleozoic rocks have been twice exposed and in- 
fluenced, 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 
haye 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 Por- 
tishead 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. 
