63(> Geolo(jical Society. 



2, ' The llhoetic and Lower Lias of Sedbury Cliff, near Chepstow.' 

 ]Jy Linsdall liichardsoii, Esq., F.G.S. 



The chief portion of the cliff-section described has a direction 

 north-east and south-west ; the dip of the beds does not exceed 3° 

 to the sonth-south-east. The section is as follows : — 



Feet inches. 

 Lower Lias... Based on a couglomerate composed of frag- 

 ments of Cothaui Marble 37 1 



jj f 2. Greenish-grey, chiefly laminated shales. 2 ft. to 3 4 



Eiiixic ^- ^-^^^^^^^'^-bed 4 to 12 



I 4. Greenish-grey shales 3 5 



( ba. Black, laminated sliales 2 4 



bh. Hard, blackisb-blue limestone b 



I 6. Black, earthy shales, full of shell-debris (> 



I 7. Hard, slightly pyritic limestone G 



I 8- 1 

 Lower | 9. |- Black, selenitic shales o 4 



<! 10. ] 



En.ETic. I 11. Firm, black, tbiuly-laminated shales 1 2 



112. Black, eartby shales G 



I 13. 1 Alternating layers of sandstone and shale, the 1 rj o 



14. J former with small quartz-pebbles J 



I 15. Sandstone (Bone-Bed), coarse and calcareous, 



i^ with masses of "Tea-green Marl ' 4 



Upper f I. 'Tea-green Marls,' witb hard bed of marlstone. 10 3 



Keupee. (^ II. Eed Marls, angular fracture 56 



3. 'Kotes on the Lowest Beds of the Lower Lias at Sedbiir}' 

 Cliff.' By Arthur Yaughan, Esq., B.A., B.Sc, E.G.S. 



The author examined this section in company with Mr. Hichardson. 

 The two chief points of interest are, the relation of the basal con- 

 glomerate to the Gotham Marble and White Lias of neighbouring 

 districts, and the examination of the faunal sequence, with a view 

 of testing the absolute vahie of ammonite-zones. The conglomerate 

 resembles the so-called ' Ealse Gotham,' and both maybe explained 

 b}' the breaking-up of Gotham Marble, in one case at intervals 

 during a continuous phase of deposition, in the other after the 

 phase of deposition which produced it had entirely ceased at that 

 place. The break thus represented in the Sedbury area may be 

 considered to correspond roughly with the time of deposition of the 

 White Lias in the areas to the south and east. The succession of 

 events appears to require a tilting, the axis of rotation being a nearly 

 east-and-west line a little south of Sedbury, with gradual and 

 uniformly-increasing depression towards the south, followed by a 

 period of horizontal equilibrium. On the other hand, the succeeding 

 P.iilonotus, Avgulatus, and Arietes-zones indicate a gradually 

 increasing depression towards the north with, a change of axis. 



A ' rang^-graph ' is given, showing the times of appearance and 

 disappearance, the abundance or rarity, of several fossils within 

 and below the zone of AmmoniUs -psilonotus^ and on account of the 

 beginning of five forms at a given horizon and the disappearance 

 of several forms immediately below it, this level is chosen as the 

 base of the zone of A. jisilonotus, rather than the point of appearance 

 of A. iilanorlis 4 feet higher up. It is hoped that the construction 

 of ' range-graphs ' will be of use in testing the value of a series of 

 ammonite-ages as divisions of relative time. 



