JURASSIC OF THE PARIS RASIN. 



585 



fossils, the sandy portions below the actual sand on the northern 

 slopes of Swindon Hill, the lower part of Shotover Hill with the 

 finely laminated clays, the uppermost part of the pit at Ely, and the 

 whole of the mass called Upper Kimmeridge in Lincolnshire *, all 

 belong to the Bolonian. In most cases no subdivision into Upper 

 and Lower can be traced ; but the record by Prof. Judd (Q. J. GL S. 

 vol. xxiv. p. 237) of Amm.gigas and Amm. Gravesianus in the " Port- 

 landian " portion of the Speeton Clay would appear to indicate the 

 distinctness of the Lower Bolonian in that locality. It appears 

 from this description that the Upper Bolonian in the northern area 

 may be divided into 3 minor parts as in the south, viz. the zone of 

 C yprina Brongniarti, the zone of Discina latissima, and the Portland 

 Sands, which last might be called the zone of Belemnites SouicJiii. 



The last series to be considered is the Portlandian. The line of 

 junction of this with the Bolonian has already been discussed. The 

 beds above this line which may be certainly placed among Jurassic 

 deposits, are the Flinty Series, the Building-stones, and the Purbecks 

 of the Isle of Purbeck. Towards the close of the Jurassic epoch 

 freshwater conditions appear to have set in in various parts of the 

 area under study. There is, however, no reason to suppose that 

 their introduction in every place was synchronous, but every reason 

 to think the contrary ; yet by the custom of calling all these beds 

 " Purbeck " their synchronism has been practically affirmed. To 

 obviate this it is proposed to include the typical Purbecks, as seen in 

 the Isle of Purbeck itself, in a " Portlandian " group, of which it will 

 form the upper member, while the Plinty series and Building-stones 

 form the lower. There can be no objection to this, if it renders cor- 

 relation easier. 



It remains, then, only to indicate the localities where these rocks 

 occur, and their position in the group. In the southern range 

 no possible representative is met with till we reach Bar-le-Duc, 

 shortly before entering the Haute-Marne. Here commences a small 

 mass of rocks formerly considered Suprajurassic, of which the most 

 noteworthy is the vacuolar oolite. From stratigraphical position, this 

 might equally well represent the upper part of the Upper Bolonian ; 

 but the occurrence in it of Astarte rugosa, a characteristic fossil of 

 true Portland rocks, leads to its being placed as Lower Portlandian, 

 a correlation to which Pellat (68) agrees (using the term in the 

 sense now defined). These beds have been proved (40) to be over- 

 lain unconformably by the Neocomian ; so that nothing higher is to 

 be found ; but it is a remarkable circumstance that the greatest 

 development upwards is just at that locality where all detrital beds 

 are at their maximum, opposite the Straits of Dijon. These beds 

 soon disappear ; and nowhere else in the same range can similar beds 

 be found. In the Charente, however, if we correlate the limestones 

 with Cyprina Brongniarti (or its representative) with the Upper 

 Bolonian, the succeeding beds with Corbula injiexa must be the base 

 of the Lower Portland. Indeed the Portlandian and Bolonian may be 



* In the map in Quart. Journ. Geol. Soc. vol. xxxi. p. 202, the names " Upper 

 Kimmeridge" and " Lower Kimmeridge" have been interchanged by error. 



