PART II. CHAPTER XV. 



191 



Area covered by Chalk Green Sand Formation. 



Geograjihical extent of White Chalk. — The area over 

 which the white chalk preserves a nearly homogeneous aspect is 

 so great that geologists have often despaired of finding any ana- 

 logous deposits of recent date ; for chalk is met with in a north- 

 west and south-east direction, from the north of Ireland to the 

 Crimea, a distance of about 1140 geographical miles, and in an 

 opposite direction it extends from the south of Sweden to the 

 south of Bordeaux, a distance of about 840 geographical miles. 

 But we must not conclude that it was ever spread out uniformly 

 over the whole of this vast space, but merely that there were 

 patches of it, of various sizes, throughout this area. Now, if 

 we turn to those regions of the Pacific over which coral reefs 

 are scattered, we find some archipelagoes of lagoon islands, such 

 as that of the Dangerous archipelago, for instance, and that of 

 Radack, with some adjoining groups, which are from 1100 to 

 1200 miles in length, and 300 or 400 miles broad ; and the 

 space to which Flinders proposed to give the name of the Coral- 

 lian sea is still larger ; for it is bounded on the east by the Aus- 

 tralian barrier, on the west by New Caledonia, and on the north 

 by the reefs of Louisiade. Although the islands in these spaces 

 may be thinly sown, the mud of the decomposing zoophytes 

 may be scattered far and wide by oceanic currents. 



Green-sand formation. — The lower division of the Cretace- 

 ous group in England is divisible, as we have already seen, into 

 Upper Green-sand, Gault, and Lower Green-sand. The green 

 grains have been found, by analysis, to consist chiefly of silicato 

 of iron, and they agree in composition with chlorite. The infe- 

 rior white marly chalk becomes more and more charged with 

 these grains until it passes into the upper green-sand, a forma- 

 tion of sand and sandy marl, frequently mixed with chert, and 

 this again passes downwards into the clay and marl, provincially 

 called Gault. Both of these subdivisions, although often dimin- 

 ishing in volume to a thickness of two or three yards, form dis- 

 tinct and continuous bands of sand and clay between the chalk 

 and lower green-sand throughout considerable tracts in England, 

 France, and Belgium ; and each preserves throughout this space 

 certain mineral peculiarities and characteristic fossils. 



The lower green-sand below the gault is formed partly of 

 green and partly of ferruginous sand and sandstone, with some 

 limestone. These rocks succeed each other in the following 

 descending order in Kent : — 



No. 1. Sand, white, yellowish, or ferruginous, with concretions of limestone and 

 chert, 70 feet. 



2. Sand with green matter, 70 to 100 feet. 



3. Calcareous stone, called Kentish rag, 60 to 80 feet * 



* FittonrGeol. Trans., Second Series, \'ol. iv. p. 319. 



