74 Rev. J. F. Blake — Form of Sedimentary Deposits. 



closely defined period, such as the Upper Llandovery beds of Wales 

 and the Shropshire area. We find that in the Malvern area the 

 May Hill Sandstone is described as entirely conglomerate and 

 sandstone, and of a thickness of at most 800 feet. At Ehayader,. 

 according to Mr. H. Lapworth,^ the same group contains 200-300 

 feet of blue shales amongst the conglomerates, and 250 feet more 

 above them. Thus, while the whole deposits have increased from 

 800 to 1,100 feet, the fine-grained detrital portion has increased 

 from nil to 500 feet. Still farther west Mr. Walter Keeping,' who 

 allowed as much as possible for folding, could not estimate his 

 metalliferous slate group at less than 2,000 feet, and in this group he 

 says that occurrences of grit, even of 2-6 inches thick, are very rare. 

 His general conclusion is, " that whilst in ... . the dawn of the 

 Silurian era the elevatory forces lifted the sea bed to form a land 

 surface over the west of England and the Welsh borders, these 

 forces influenced the greater part of Wales only in a less degree," 

 and did " not interfere with the continuous deposit of sediment." 

 In other words, he recognizes that these deposits are thickest, 

 independently of their character, at the greater distance from land. 



Take, again, the case of the Permians on the east side of the 

 Pennine axis. They were deposited after the elevation of this axis, 

 and towards the south end of their range there are indications of the 

 shore-line both in the basal breccias and in the Magnesian Limestone^ 

 Near this shore-line the three members there found have a total 

 thickness of 80-160 feet ; but thirty miles away in the Searle 

 boring the same beds (excluding higher ones there represented) 

 have increased to 360 feet, to which increase the Magnesian Lime- 

 stone makes only a negative contribution. 



More interest in this regard attaches to the Jurassic rocks, as 

 their thickness and character is more widely observable, and 

 also because it was to these that Professor Hull appealed, on the 

 assumption (for no shore-line was proved) that their " attenuation 

 is due to the increase of distance from the sources of supply," to 

 prove that their " sedimentary materials .... have com© 

 from land occupying the region of the North Atlantic." ^ 



The borings that have been made at Mickleton, Burford Synett, 

 Wytham, and Turnford have certainly demonstrated the thinning out 

 of all the Jurassic series and the Keuper towards the south and 

 east ; and as the slope thus produced is not greater than the angle of 

 rest of the materials, it is competent for anyone to say that all 

 the beds are in the position in which they were laid down (vertical 

 elevation of the whole excepted), and that the sea bed beyond them 

 to the east was too far removed from land to receive any of 

 these terrigenous deposits. This, indeed, is what must be said if 

 deposits thin out gradually seawards. But in this case we throw 

 over the whole argument for a ' Palaeozoic ridge,' and leave the- 

 fresh-water Purbecks of Buckinghamshire wholly unaccounted for. 



1 Q.J.G.S., vol. Ivi, pp. 69-135. 



2 Q.J.G.S., vol. xxxvii, pp. 141-150. 



3 Q.J.G.S., vol. xvi, p. 80. 



