Vol. 52.] IN YORKSHIRE AND LINCOLNSHIRE. 217 



However, where so much uncertainty still exists, it appears to 

 me that, pending the accumulation of further evidence, we shall 

 scarcely be justified in pronouncing on one side or the other a final 

 decision in this matter. Even with regard to the rocks of this age in 

 the South of England, on the classification of which the whole of this 

 discussion hinges, there is still much obscurity. It seems now to be 

 very generally acknowledged that there is a passage from Purbeck to 

 "Wealden where both are fully developed 1 ; and recently even the 

 strength of the evidence on which the Wealden itself is classed as 

 Cretaceous has been challenged, and the chief elements of its fauna 

 declared by several authorities to be Jurassic rather than Cretaceous 

 in their affinities. 2 



It appears, in short, to be the fact that while over Western Europe 

 there is usually a distinct stratigraphical break at the base of the 

 Upper Cretaceous, as Mr. Strahan and others have frequently 

 pointed out, 3 the base of the Lower Cretaceous, as at present 

 recognized, whether the sequence be freshwater or marine, presents 

 no such break, but a more or less gradual passage both in character 

 of deposit and in fauna. Under such conditions it is mainly a 

 question of general convenience what particular horizon shall be 

 taken as the boundary of the systems, and the essential determi- 

 native must rest in the agreement of competent opinion. 



VII. Concluding Summary. 



The leading conclusions of this paper may be epitomized as 

 follows : — 



1 . Further work on the Speeton section, while extending our know- 

 ledge of the palaeontological details, has fully sustained the 

 results of the author's previous investigations. 

 2. The evidence at present available is insufficient to demonstrate 

 the exact conditions which bring about the rapid attenuation 

 and final disappearance of the Speeton Series in a westerly 

 direction in Yorkshire. Contrary to the accepted view, how- 

 ever, the lower zones are probably the first to die out, and 

 are overstepped or overlapped westward by the higher divisions, 

 since at Knapton, 14 miles inland, only the upper zones of 

 the coast-section can be proved to occur, as shown by the 

 presence of the marls with Belemnites minimus passing upward 

 into the Red Chalk, and by the fossils in the old collections, 

 including Hoplites Deshayesii, under the name of Ammonites 

 Tcnajitonensis, Bean MS., and a few others of the same zone. 



1 See H. B. Woodward in Mem. Geol. Surv. 'Jurassic Eocks of Britain,' 

 vol. v. pp. 3 and 243 et seqq. (with good bibliography). 



2 O. 0. Marsh, Geol. Mag. 1896, p. 8 ; [also in « Nature,' vol. liii. (1896). 

 p. 436, as regards the reptiles ; A. S. Woodward, Geol. Mag. 1896, p. 70, as 

 regards the fishes; and A. C. Seward, in 'Nature,' vol. liii. (1896) p. 462, as 

 regards the plants]. 



3 See, among others, A. Strahan's recent paper ' On Overthrusts of Tertiary 

 date in Dorset,' Quart. Journ. Geol. Soc. vol. li. (1895) p. 561. 



