Vol. 60.] SECTIONS IN THE BRISTOL DISTRICT. 187 



Gotham Marble was laid down in very shallow water. Only after the 

 White-Lias period did the water finally become moderately deep. 

 We may conclude, then, that the Rhsetic Series was laid down 

 in a gigantic shallow lagoon connected with the open 

 sea to the south. 



The waters were probably brackish on the whole, but with great 

 variations at different times and places. Such extensive sheets of very 

 shallow water must have been extremely apt to dry up whenever the 

 sun was hot, especially in isolated pools. It would in these become 

 very saline ; the animal life would die, and rock-salt and gypsum 

 would be deposited. As Ramsay remarks, the Rhaetic Beds do 

 occasionally and very locally contain gypsum and pseudomorphs 

 after rock-salt. Around Bristol baryto-celestine is common, which 

 is here a mixture of sulphates of barium, strontium, and calcium, 

 and probably results from precipitation in a concentrating lagoon. 



On the other hand, in many pools fed rather by streams and rain 

 than by the open sea, the water would be brackish or even quite 

 fresh. Prof. T. Rupert Jones claims that Estheria mhmta and 

 Banuinula were fresh- or brackish-water forms. Probably Naiadita 

 only flourished in fresh or slightly saline-water. 



It will be seen from the foregoing remarks that I do not believe 

 that the Rhaetic Beds were laid down in an estuary, and that for the 

 following reasons : — 



(a) — They obviously follow the distribution of the Triassic lake 

 pretty closely, as they occur at places so far apart as Gainsborough, 

 Uplyme in Dorset, Watchet, Penarth, Burton-on-Trent, and along 

 the Jurassic escarpment. It would have been indeed extraordinary 

 if in so short a time an enormous river had developed, the estuary of 

 which should correspond so ciosely to the Triassic lake. The shape 

 of the area over which the various Rhaetic exposures are distributed 

 does not at all suggest an estuary. 



(6) — The principal evidences suggesting an estuary in geology 

 are the abundance of land-plants and animals, and the presence of a 

 brackish-water fauna. Xow, the Rhaetic Beds do not display these 

 at all well. The only certainly land-derived remains are Microlestes, 

 which is found in the infra-Bone-Bed Series, the wings of insects, 

 and a few pieces of drift-wood, some of which I have recorded from 

 Redland. Most of the fossils are not brackish-water, but oceanic. 



IV. The Stratigraphy of the Rh^tic Series. 



Zoning of the Rhaetic Beds in England. 



In 1861, Moore, in his classic paper (4), zoned the series between 

 the Keuper and the Planorbis-heds as follows (op. cit. p. 487) : — 



(Plan orb is-Beds. ) 

 Enaliosauriau Zone. 

 White Lias. 1 -& 



Avicula-contorta Beds. J 

 (Keuper.) 



It is curious that no more definite palaeontological zones should 



