268 [Proc. B. N. F. C, 



thickness, and also in Northern Europe to a less extent. It 

 would be no exaggeration to say that these Cretaceo-Eocene 

 formations may have reached 10,000 feet in thickness. Geo- 

 logists have not sufficiently realised the importance of the gap, 

 and when beds are discovered unmistakably belonging to this 

 middle age, the question as to whether they are Cretaceous or 

 Eocene is hotly disputed. This is especially the case when 

 their fossils include plant remains. In England, the Chalk is a 

 purely oceanic deposit, containing no indications of the proxi- 

 mity of land, while the Eocenes are fluviatile or delta deposits. 

 How the Chalk area became converted from ocean to land, and 

 for how long a time it had been land before the deposition of 

 the Eocenes commenced, are questions that can only be solved 

 by a study of the rocks of remote countries. Its denudation 

 had proceeded for ages, and on a colossal scale, before the de- 

 position of the Eocenes commenced, since even their lowest 

 beds consist of extensive tracts of flint ground into pebbles and 

 sand. But the vastness of the interval is chiefly to be inferred 

 from the striking and complete changes that took place in the 

 fauna or floras of our area in it. Throughout the whole of the 

 Eocenes, the progressive change is trifling, and difi'erences seen 

 in the forms of life are chiefly due to migration through changes 

 of temperature. Throughout the Cretaceous period too, the 

 changes in the types of fauna are so slight as to be little more 

 in value than are commonly recognised as specific. Each 

 period, notwithstanding the breaks that occur, has its continuous 

 and characteristic faunas and floras. The Cretaceous and Eocene 

 faunas, on the contrary, seem to possess nothing in common, 

 and present, perhaps, the most remarkable gap in the geological 

 record. How or when the Cretaceous fauna disappeared, we 

 cannot trace in our area, yet we know its extinction must have 

 been gradual, for Cretaceous types lived on elsewhere, mingled 

 with mollusca of a strikingly Eocene facies. A large number 

 of localities in Europe, North America, and New Zealand have 

 yielded floras of completely Eocene aspect, interstratified with 

 beds containing groups of mollusca, some of which, at least in 



