T1IK JURASSIC SYSTKM 387 



\Vith respect to tin- t'pper Lias there has been much discussion 

 and difference of opinion as to the horizon at which the line should 

 be drawn between the Lower- and Middle Jurassic Series. Where 

 the two series are fully developed, as in the south-west of England, 

 there is often a considerable thickness of sand between the Lias 

 clays and the limestones of the Inferior Oolite, and as the sands 

 were clearly passage-beds, some have included them in the Upper 

 Lias, and others have grouped them with the Inferior Oolite. 



The recent zonal work of Mr. S. S. Buckman has made it clear 

 that the sands are not always on the same zonal horizon, but may 

 be of any age from the Lilli to the aalense zone. It is also clear 

 that they pass laterally into Upper Lias clays, and that they are as 

 a whole more closely connected with the Lias than with the over- 

 lying Inferior Oolite. Moreover, in Yorkshire there is a distinct 

 stratigraphical break between the two series, and it occurs at the 

 base of the ''Dogger" sandstone which overlies the zone of 

 Grammoceras aalense. Hence it is convenient to place the divisional 

 line at the summit of the zone of Gram, aalense, and to consider 

 that of Lioceras opalinum as forming the base of the Middle Series. 



By adopting this line we are in agreement with the Committee 

 who drew up the scheme of the International Geological map of 

 Europe, and in England it includes all the sands in the Lias except 

 about 5 feet at the top of them near Bridport In some localities 

 several zones are condensed into a limestone bed of a few feet or 

 even inches in thickness, and this happens sometimes with the 

 lowest zones and sometimes with the highest. 



Fossils of the Lower Lias 



Adinozoa. Montlivaltia rugosa, Heterastrea Tomesi. 



Echinoderma. Pentacrinus briareus, Isocrinus basaltiformis, Cidaris 



Edwardsi. 

 rachiopoda. Spiriferina Walcotti, Rhynchonella variabilis, R. rimosa, 



Waldheimia (Cincta) numisrnalis. 

 Lamellibranchia. Pteria (Oxytoma) cygnipes, Cardinia Listeri, Grypluea 



arcuata (^incurva), Hippopodium ponderosum, Lima 



( Plagiostoma) gigantea, Pseudomonotis decussata, 



Ostrea liassica. 



