1857.] GODWIN-AUSTEN — BOULDER IN CHALK. 259 



perse the coarser materials of that zone beyond the limits of depth 

 which have been here indicated. 



Part II. 



Mode of deposit, extent, and nature of the Fossils of the White 

 Chalk. — It next remains that we should consider what are the con- 

 ditions which are indicated by the White Chalk. 



Two naturalists lately lost to us, and both well fitted to give an 

 opinion on such a subject — A. d'Orbigny and E. Forbes, have 

 said that the pure White Chalk must have been the deposit of a 

 deep and open sea. Apart from the evidence to be derived from its 

 included animal remains, there is also that of its mineral composition, 

 dependent on the specific gravity of its component particles, and 

 which shows that it belongs to the extreme outward zone of distri- 

 bution, as compared with the sand-zones, or those of ooze or mud. 

 In these fine sedimentary beds, and in the isolated blocks and shingle 

 which they occasionally include, we have the two terms of the series 

 of mechanical products of all seas and of all periods, namely the 

 abyssal and the marginal ; and from the presence of these two we may 

 feel assured of the co-existence of every other intermediate form of 

 sea-bed. The White Chalk had its equivalents of mud-depths and 

 sand-zones ; and its own remarkable extent as a deposit is due solely 

 to the peculiar condition of the ocean of that time, which furnished 

 so much calcareous material for abrasion and removal. Systematic 

 geologists have been so disposed to describe the White Chalk as an 

 independent portion of the cretaceous series, that the occurrence of 

 these extraneous materials is of as much use in showing that it is 

 merely one subordinate member of an assemblage of marine deposits, 

 as the materials themselves are in indicating the physical and geo- 

 graphical arrangements of that remote period. 



The area over which that uniform deposit of pure calcareous 

 matter known as chalk was deposited may be accurately defined. 

 It will be unnecessary to follow it over the whole of its extent ; but in 

 Western Europe it included the south-eastern half of Britain — from 

 Devonshire to Yorkshire as far as a line much in advance of the present 

 chalk-escarpment. In Denmark it occurs in Laelland and Moen, 

 whence it trends south-east, across Prussia and Northern Russia. 

 In France the area of true White Chalk extends south from Cal- 

 vados to the Loire, and from the whole of the Paris Basin eastwards ; 

 in Belgium it appears under precisely the same characters as in France 

 and England. Beyond this area, the deposits which were synchro- 

 nous with the White Chalk put on other mineral characters. 



In every sea or ocean the zones of sedimentary deposit hold a 

 general parallelism to one another, so that if the form or direction 

 of one can be ascertained, that of the rest necessarily follows. It is 

 by the evidence of the included fauna that we are certified that dis- 

 tricts surrounding the area which we have here defined were its equi- 

 valents in the Cretaceous series. For the present, the form of the 

 area of deep and open sea deposits during the period of the Upper 

 Cretaceous series may be taken as here drawn. 



