Vol. 59.] OF THE LOWER LIAS AT SEDBURY CLIFF. 397 



slightly rounded at the ends, and the matrix is usually of the same 

 texture as the fragments which are embedded in it. 1 



The similarity of the two rocks can be completely explained on 

 the supposition that both have been formed by the breaking-up of 

 Cotham Marble 2 ; while the differences seem to accord with the 

 assumption that the ' False-Cotham ' was formed by the partial 

 breaking-up of the Cotham Marble at intervals during a continuous 

 phase of deposition, whereas the Sedbury-Cliff conglomerate was 

 formed by the complete break-up of the layer of Cotham Marble 

 after the phase of deposition which produced it had entirely ceased 

 at that place. 



We have, then, evidence that a rock-layer lithologically similar 

 to the Cotham Marble was laid down in the Sedbury area, but 

 subsequently broken up and cemented into a conglomerate. 



The time occupied by the hardening of the Cotham layer, its 

 destruction, and subsequent cementation into a conglomerate may 

 be considered to correspond roughly to the time of deposition of the 

 White Lias in the areas on the south and east. The Kelston-Station 

 cutting, on the Midland Railway between Bristol and Bath, lies 

 some 20 miles from Sedbury Cliff in a direction south 30° east ; and 

 the Stoke-Gifford cutting, on the new South Wales Direct Line, lies 

 almost exactly halfway between the two. At Kelston Station there 

 is, above the Cotham Marble, a considerable thickness of White 

 Lias, capped by the thick Sun-Bed 3 ; at Stoke Gifford, the Sun-Bed 

 lies immediately upon the Cotham Marble 4 ; while at Sedbury Cliff 

 the White Lias is entirely missing, and is replaced by the con- 

 glomerate, made up of fragments of Cotham Marble. The section 

 at Redland (a suburb of Bristol) supplies a link between the 

 Kelston and Stoke-Gifford sections, for at that place there is less 

 than 2 feet of rubbly White Lias between the Sun-Bed and the 

 Cotham Marble. 5 In the cutting south of Chipping Sodbury, on 

 the South Wales Direct Line, situated about 7-J miles east of Stoke 

 Gifford, the White Lias is (as at Stoke Gifford) represented only by 

 the Sun-Bed, which rests immediately upon a precisely similar layer 

 of typical Cotham Marble. 6 



Without attempting any final explanation of the exact conditions 

 of deposition which resulted in the production of the Cotham 

 Marble, it may, with great probability, be assumed that these 

 conditions were practically identical wherever the rock is found. 

 In other words, it seems probable that the whole area over 

 which the Cotham Marble extended was, simultane- 

 ously, at approximately the same depth and subject to 



1 I am much indebted to the kindness of Mr.W. H.Wickes for the opportunity 

 of examining several fine specimens of ' False-Cotham ' from Redland, Stoke 

 Gifford, and Aust Cliff. 



2 The suggestion that ' False-Cotham ' was formed by the breaking-up of 

 Cotham Marble was first made by Mr. A. Rendle Short, B.Sc. 



3 Proc. Bristol Nat. Soc. vol. x (1901) p. 35. 



4 Quart. Journ. Geol. Soc. vol. lviii (1902) p. 727. 

 s Proc. Bristol Nat. Soc. vol. x (1901) p. 38. 



6 Quart. Journ. Geol. Soc. vol. lviii (1902) p. 719. 



