INTRODUCTION. XI 



fossils of the Wealden, Urgonian, and Aptian groups are included 

 and not distinguished." l 



It will be well, therefore, at the outset to state definitely in 

 what sense the term "Wealden is used in the present Catalogue. In 

 his Memoir on the Weald, Topley expresses the opinion that the 

 "Wealden and Purbeck strata should be classed together as one 

 unbroken series. 2 This and similar statements by various geolo- 

 gists might be quoted in support of a Purbeck- Wealden series, 

 and, possibly, such would best represent the true stratigraphical 

 relations of the two sets of beds. On the other hand we are not 

 concerned here with any critical examination of the geological 

 evidence, whether stratigraphical or palseontological ; and for the 

 present, at least, it will be more convenient to conform to the 

 general usage of the term by English geologists, and include 

 the Wealden beds in the Cretaceous system to the exclusion of 

 the Purbeck. 



In the Report of the British Committee to the International 

 Geological Congress of 1888, the Reporters, Messrs. A. J. Jukes- 

 Browne and W. Topley, 3 recommended a general classification of 

 the Cretaceous system, in which the " Lower series " of the 

 system is defined as comprising the Lower Greensand (Yectian), 

 Weald Clay and Hastings Sands; the two latter being bracketed 

 together as Keocomian. 



In the correlation table of the Cretaceous system, given by the 

 same authors, the Purbeck beds are placed at the base in the 

 South-Eastern area. 



In Geikie's Text-book 4 the usual classification is adopted, the 

 Purbeck beds being regarded as the uppermost members of the 

 Jurassic system. 



In this connection it may be of interest to quote the views 

 recently put forward by Messrs. Pavlow and Lamplugh in their 

 Essay on the Speeton Clay. Their correlation of four zones 



1 Potomac Flora, p. 332. 



2 Weald, p. 321. 



3 Cong. Geol. Int. App. B. Cretaceous, p. 77. 



4 Text Book of Geology, 1893, p. 938. 



