398 ME. A. VATJGHAN ON THE LOWEST BEDS [Aug. I9O3, 



the same type of deposition: this may be expressed by saying 

 that the whole area was in horizontal equilibrium. 



The actual limits of the area covered by the Cotham Marble 

 cannot be definitely ascertained, but it certainly extended south- 

 ward into the Radstock area. 



At Sedbury Cliff the deposition of the Cotham Marble must have 

 been succeeded by an elevation of the floor, which produced the 

 breaking-up of the Cotham-Marble layer in situ. 1 It seems to me 

 improbable that the Cotham-Marble deposit indicates any consider- 

 able depth of water, for, in the ' False-Cotham,' we have apparent 

 evidence of one or more interruptions, when the layers already 

 formed were partly broken up, after which the conditions of depo- 

 sition were immediately resumed. 



Farther south the Cotham Marble is immediately succeeded by a 

 fine-grained, slabby, impure limestone (the White Lias) which 

 increases uniformly in thickness as far as the Eadstock area. 

 The constitution of the White Lias is practically the same as that 

 of the Cotham Marble, and consequently implies little alteration in 

 the manner of deposition. 



If we imagine a gradual tilt of the horizontal floor to take place, 

 immediately after the Cotham-Marble deposition, and to have been 

 so performed that the axis of rotation was a line running a little 

 south of Sedbury Cliff, from west slightly south to east slightly 

 north, the result would be a gradual and uniformly-increasing 

 depression towards the south, and an elevation towards the north. 

 If, further, the rate of deposition towards the south approximately 

 kept pace with the rate of depression, we should obtain a result 

 exactly satisfied by all the conditions of the problem. This phase, 

 characterized by gradually-thickening deposition towards the south 

 and actual destruction of deposits towards the north, was succeeded 

 by a period of equal rate of deposition over the entire area, for the 

 Pleuromya-Be&s (which succeed the White Lias towards the south, 

 and lie upon the conglomerate at Sedbury Cliff) exhibit a remark- 

 ably-uniform lithological aspect throughout the area, and contain 

 almost precisely the same fauna, with the same relative vertical 

 distribution, whether they are studied at Kelston, at Eedland, at 

 Stoke Gifford, at Sodbury, or at Sedbury Cliff ; the actual thickness 

 of the beds is also very nearly the same throughout the area. 



Here, then, we have a second period of horizontal 

 equilibrium. 



The higher beds of the Lower Lias, which compose the Psilonotus-, 

 Angidatus-, and Arietes-zones, 2 point to a change of axis of rotation 

 and reversed oscillation ; for they exhibit gradually-increasing 



1 The large, slab-like fragments, the angles of which are frequently quite 

 sharp, prove conclusively that the conglomerate was made by the breaking- 

 up of material on the spot, and not of material brought from any distance. 



2 An explanation of the connotation of these zonal terms is given in my 

 paper on the Lias of Keynsham, Proe. Bristol Nat. Soc. vol. x (1901) pp. 14 

 et seqq. 



