260 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Dec. 16, 



The period of the Upper Chalk and its equivalents corresponds to 

 that of the greatest extension of the area of the cretaceous sea or 

 ocean ; and the only means we have by which to define the extreme 

 boundary-line of this area is by following out the gradation of sea- 

 bed in the direction in which it passes into sands and coarser accu- 

 mulation, but preserving at the same time its characteristic fauna. 

 Marginal beds of this age occur on the southern portion of Norway 

 and Sweden. In Westphalia and Rhenish Prussia, sands are the 

 equivalents of White Chalk. On the west of the European area the 

 cretaceous strata of Valognes, though composed of sand and siliceous 

 conglomerate or shingle, are yet Upper Chalk. Such is also the case 

 with the sands of the West of England, which form the extreme limit 

 of the cretaceous series about the Bovey Y alley. In this way not only 

 the form and extent of the cretaceous ocean in its greatest range, but 

 also the character and composition of its coast- line, can be determined 

 with approximate certainty, for a given number of points. 



Belemnitella mucronata, which makes its first appearance in our 

 Upper White Chalk, marks the equivalent of the White Chalk in 

 other places, whatever the mineral composition of the beds may be. 



I have elsewhere shown* what was the extent of that first portion 

 of the cretaceous sea which gradually extended itself from the Medi- 

 terranean area, across France, or what is now part of the south-east 

 of England. From this first stage, the history of the cretaceous for- 

 mation consists of a progressive increase of area in water, together 

 with a change in time of its inhabitants or fauna ; so that, over the 

 whole of the cretaceous area, the lowest beds show littoral or shallow- 

 water conditions, both mechanically and zoologically : and what is 

 known as the grouping of forms characteristic of the great subdivi- 

 sions of the formation disappears when the marginal beds are carried 

 out to their widest range or limit, and holds good only for the central 

 portions of the area, where the series of depositions represents both 

 the changes which are due to the successive conditions of depth as 

 well as those which animal forms exhibit in time, and where conse- 

 sequently the results of these changes are presented in order of 

 superposition. 



In this way it happens that the assemblage which figures in our 

 lists as the fauna of the White Chalk is truly that of the upper or 

 newest cretaceous period, — but in part only, and is not characteristic 

 of those marine conditions of which that deposit is the result. The 

 remains met with in the pure White Chalk area belong for the most 

 part to a much higher sea-zone. This view of the character of the 

 remains met with in the Chalk must have often suggested itself 

 to naturalists who may themselves have collected from that forma- 

 tion. Thus the stony remains of the Anthozoa have all been broken 

 off at the base, and are without any support in the beds in which 

 they occur. The Bryozoa belong to a condition of sea-water very 

 different from that of those calcareous mud-beds. The Echinoderms 

 show characters of having undergone decomposition. Again, the 

 most common and characteristic forms of the White Chalk are frag- 

 * Quart. Journ. Geol. Soc. vol. xii. p. 68. 



