Vol. 60.] SECTIONS IN THE BRISTOL DISTRICT. 185 



yellow, or grey clayey beds above the shaly Naiadita -limestones, 

 and below the Cotham Marble. Those beds are almost invariably 

 barren of organic remains. 



(//) The Cotham Marble. — I have discussed the origin of this 

 rock elsewhere (53), and merely state here that my theory demands a 

 chemical origin, by precipitation of carbonate of lime, under a hot 

 sun, in an extensive, very shallow lagoon with occasional deeper 

 pools. 



The Cotham-Marble horizon, when that stone is absent, is usually 

 represented by white rubbly limestone, or else yellow clay or shales. 

 Sometimes Chemnitzia nitida and Monotis decussately the most 

 characteristic representatives of the Cotham-Marble fauna, occur 

 in these, as at Pylle Hill (36), where Wilson subsequently found 

 the Landscape-Stone in his bed N (45). 



(h) The White Lias. — With this may be here included all 

 the beds between the Cotham Marble and the lowest Ammonites 

 planorbis or Am. torus. 



The White Lias itself usually consists of rubbly, white, cal- 

 careous beds, with a good deal of siliceous matter. The occurrence 

 of insects in these shows that they were laid down in shallow 

 water (1). They are, as a rule, shelly. 



Above these, beds exactly like those of the overlying Liassic lime- 

 stone (that is to say, thinly-bedded, blue shelly limestone weathering 

 yellow or brownish-yellow, with yellow or brown shaly partings) 

 generally occur. Sometimes, as at Aust and Stoke Gifford, these 

 follow immediately upon the Cotham Marble, with at most a shaly 

 parting, but no rubbly limestone. 



Around Bath is sometimes found the stratum, called the Sun- 

 Bed, which Moore and others take to mark the upper limit of the 

 Rhastic. It is hard, with conchoidal fracture, very fine-grained, 

 cream-coloured, blue, or white ; the upper surface is corrugated, 

 and some consider it sun-cracked. It is marked by worm-tubes, 

 which are very common in the White Lias in some places (as, for 

 example, at Bedland), and contains Modiola minima and Ostrea 

 liassica. I think that to this bed, which is extremely local, has been 

 accorded an importance which it does not deserve. Its chief 

 interest, in conjunction with the occurrence of insect-wings in the 

 White Lias, is the evidence provided of the persistence of the 

 shallow-water conditions that prevailed throughout the Ithsetic 

 Period until this time. 



(B) A General Account of the Physical Geography of the 

 Rhsetic Period in England. 



The most important paper that has appeared on this subject was 

 by Sir Andrew Ramsay in 1871 (16). He argued that the Keuper 

 was laid down in a great inland sea, which gradually dried up, 



