8 PROCEEDINGS 0:F THE GEOLOGICAL SOCIETY. [IToV. 8 



It maybe inferred from Sir H. De la Beche's description that he 

 had not had opportunities of distinguishing the two surfaces of plant- 

 growths, nor the remarkable floor of angular materials on which the 

 forest-bed is rooted. It is not stated whether the " silt and sand " 

 of the " inclined plane " was freshwater or marine. 



The succession of changes here indicated corresponds with that of 

 numerous other localities at which I have had opportunities of ex- 

 amining the evidence in the West of England. The Porlock example, 

 in addition, seems to fix a relative date for a part, namely, as being 

 subsequent to the great subaerial weathering of the surface during 

 the Glacial Period. The elevation of the land was greater at that 

 time than at present, but by how much we have no means of 

 determining. 



The diff'erence of level within which the land has oscillated since 

 then need not be estimated at more than 40 feet, the range between 

 high and low water on the Bristol Channel coast being taken at 33 

 feet. It is certain that such forest-growths as pass on all sides 

 beneatJi the line of low water could not have lived at the high-water 

 level, but a rise of 50 feet would convert the upper part of the Bris- 

 tol Channel into land-surface. The greatest depth at which sub- 

 merged land-surface has been ascertained is about 120 feet ; a rise 

 of such amount would place the whole of the Bristol Channel in the 

 condition of dry land, and such probably it was at the time of the 

 forest-growths. 



The line of the Bristol Channel is that from which the amount of 

 depression of the British area in the West increased progressively 

 northwards, corresponding to the line of the Thames valley in the 

 East. There is clear evidence that the line of the English Channel 

 was occupied by sea during the " Cold Period" (Quart. Journ. Geol. 

 Soc. vol. vii. p. 135) ; and at the Newcastle Meeting of the British Asso- 

 ciation I indicated to what extent this unsubmerged area of the South 

 of England was affected by that depression. Like evidence may 

 now be derived from the coasts of IS'orth Devon and Somerset. 



The lowest and oldest beds beneath the Bridgewater levels are of 

 sands and subangular gravel, made up of all the materials of aU the 

 hiU-ranges which send their streams towards that depression, from 

 the grauwacke of Exmoor and the Quantocks up to the Chalk 

 inclusive. 



From the great extent of the old alluvia of this area of drainage, 

 the volume of the rivers must at some time have been very great, and 

 as no part of it was comprised by the line of circumpolar submer- 

 gence, these alluvia are referable to the subaerial conditions of the 

 whole of the Glacial Period. The great gravel-beds of the Bristol 

 Channel, noticed by Dr. Buckland as proofs of his Diluvial theory, 

 are the accumulated glacial alluvia of all the rivers of the West, from 

 the Severn to the Tone : they are the equivalents in age of the great 

 accumulations of angular debris. 



Over the whole of the West of England the remains of the great 

 Pachyderm fauna occur abundantly in, more generally beneath, the 

 old alluvia, as also beneath angular debris, at various elevations. 



