part 4] 



JURASSIC CHROIS'OLOGY. 



397 



Day's Details. 

 Strata. Fauna. 



The Starfish-Bed. Ophioderma. 



The Shell- Bed 

 immediately 

 underlies tlie 

 Starfish- Bed. 



Mudstone, with 

 nodular concre- 

 tions and shells. 



Maris, with a 

 layer of small 

 nodules. 



The Three Tiers. 



Rich in Conchi- 



fera. 

 Ammonites 



margaritatus. 

 [^Amaltlieus spp.1 

 A. thouarsensis. 

 [Seguenziceras 



cf. algovianuml]^ 



Ammonites of 

 several species. 



A. margaritatus. 

 A. loscomhii. 

 A. fimhriatus. 



S. S. Buckman's Details. 

 Strata. Faun a. 



Blue somewliat marly Ophioclerma. 

 sandstone. 



The black nodule-bed. 

 (Bluish marly stone, 

 with black nodules.) 



Light-blue clay. 



Dark blue conchoidal 

 clay, with two lines 

 of small nodules near 

 the top. 



The Three Tiers form 

 ' The Ledges,' which 

 show on the foreshore 

 under the eastern end 

 of Thovncombe Bea- 

 con at low tide. (Hard 

 blue sandstone.) 



Amaltheids. 



Fragmentary Amal- 

 theids in the nodules. 

 Body-chambers of 

 Amaltheus aff. cleve- 

 landicus, Ammonites 

 cf. hoscensis and like 

 forms in the clay in 

 line with the nodules 

 {Tragophylloceras 

 afF. loscombei J. Sow- 

 erby sp. [derived?]). 



Most of the hard beds mentioned in Table I are separated by 

 thick masses of nnfossiliferous or poorly fossiliferous sands or clays 

 (see Day). From the Junction-Bed to his ' blocks of indurated 

 sand,' which I take to be the T.-thonicomhieusis Bed, Day makes 

 18 feet at Down Cliffs and from the Junction-Bed to the Marlstone- 

 Bed92|feet; whereas my measurements at Thorncombe Beacon 

 give 8 and 68 feet respectively. More will be said about this 

 difference presently, for it is a rather interesting point. 



In certain pebbles of blue sandy matrix I found small Amaltheids, 

 which are very similar, if not identical, with Amaltheits IcGvis 

 (Quenstedt), and their matrix certainly recalls that of the Scalpa 

 Sandstone. In my first paper on Jurassic Chronology I proposed 

 a zone of Amaltheus Icevis^ placing it directly beneath that of 

 Paltopleuroceras spinatum. Kecent researches have tended to 

 confirm the value of the zone, but have raised considerable doubt 

 as to its position. According to the Thorncombe evidence the first 

 zone below that of P. spinatum is that of the so-called Phynchonella 

 northamptonensis {Tetrarliynclxia tlioriicomhiensis) and the next 

 is that of the margaritatus bed with large examples of Amal- 

 theus, The specimens supposed to be A. laevis may come from the 



I, 7, p. 262. 



2e2 



