184 ME. A. RENDLE SHORT ON RH.ETIC [^ a 3" I 9°4' 



(d) The Pec^H-Limes tones. — Just above the Black Shales it 

 is usual to find one or more seams of very hard, thick, badly -jointed 

 shelly limestone, usually blue inside and yellowish-brown when 

 weathered, rich in Pecten valoniensis. There are frequently two 

 such bands, and occasionally, as at Aust and Watchet (6), three. 

 Where absent, they are represented by a shale very rich in Pecten 

 valoniensis, which is also found always in the Black Shale immediately 

 beneath them. These limestones are not usually ripple-marked or 

 sun-cracked ; they are probably the relics of an ancient shelly ooze, 

 deposited in rather deeper water than the rest of the Rhaetic. The 

 transition from shale to limestone is due to the following of a 

 dry period, with less washing down of mud by the streams, upon a 

 wet one ; and at the same time to the importation of many molluscs, 

 and very likely minute calcareous organisms as well. 



(e) The Naiadita -Beds. — Above the Pec^i-Limestones occurs a 

 series of thinly-bedded limestones and calcareous shales, originally 

 blue when massive, but weathering yellow or grey, containing 

 Estheria minuta, var. Brodieana, and Naiadita. These beds are 

 recorded at most of the important Rhoetie exposures. 



They are seldom very fossiliferous, and are not at all like the 

 shelly P^eu-Limestones. Nearly all, usually all, their fossils occur 

 along certain shelly or plant-covered horizons. Otherwise, they are 

 fine-grained and barren. It is interesting to note how Estlieria, 

 Naiadita, and the shells {Pecten, Axinus, Cardium, etc.) tend to 

 avoid each other's horizon. I have never seen Estlieria and 

 Naiadita together; shells and Naiadita only occur in company 

 occasionally. 



Most noticeable about these beds is the abundant and often 

 striking evidence of shallow-water conditions, or even exposure, 

 that they commonly afford. At Redland this is excellently shown 

 by a fine series of ripple-marks, sun-cracks, and worm- tracks. It is 

 common to find worm-tracks on a rippled surface, in a grey thinly- 

 bedded marlstone, looking exactly like a modern beach. I have 

 found specimens of sun-cracked argillaceous limestone showing 

 most excellently the fine flaking-ofT of the top layers of dried mud. 

 Further evidence of shallow water is given by the frequency of 

 wings of insects, as, for example, at Stoke Gifford and Bedland. 

 These shallow-water conditions prevailed over an extensive area, 

 not merely along a shore-line. They are well marked at Redland, 

 Stoke Gifford, Penarth, Watchet, etc. 



The chemical composition of these beds is variable. One specimen 

 from Redland, more calcareous in appearance than most, contained 

 60 per cent, of carbonate of lime. Others contain less. I found 

 on microscopical examination only tiny calcite-crystals and shell- 

 fragments, but no foraminifera. It is true that the examination 

 was a somewhat imperfect one. 



(/) The Clay-Beds. — In practically every British Ilhaotic 

 section there is a variable but often considerable thickness of blue. 



