152 J. V. ELSDEN AGEICTJlTtTEAL 



Royston, superficial deposits almost universally alter the agricultural 

 features of the surface. Much of the western part of this chalk 

 district is covered by a stiff flinty loam or clay, which has probably 

 resulted from a disintegration and solution of the chalk, by the aid 

 of water and carbonic acid, leaving behind the flints and the 

 insoluble clay present in the chalk. The origin of this " clay-with- 

 flints," as it is called, is especially interesting both from an 

 agricultural and a geological point of view ; for when we examine 

 analyses of chalk from different localities we are struck with the 

 small percentage of alumina present. 



TABLES SHOWING PERCENTAGE COMPOSITION OF CHALK. 



Upper Chalk (Norfolk).* Upper Chalk ( Hampshire).! 



(Playfair.) (Way and Paine.) 



Calcium carbonate 95 



,, sulphate 



,, phosphate 



Magnesium carbonate 



Sodium chloride 



Alumina 



Protoxide of iron 1 



Silica 



Moistm-e 



50 Sand and insoluble matter "66 



37 Carbonic acid 42-98 



10 Sulphuric acid trace 



19 Phosphoric acid -08 



13 Chlorine none 



64 Lime 55'24 



70 Magnesia '10 



51 Potash -06 



70 Soda -14 



Oxide of iron and alumina '74 



When we consider, therefore, the large amount of chalk which 

 must have been dissolved away before a thickness of even a few 

 feet of this flinty clay could accumulate, we cannot but be struck 

 with the immense time during which the slow process of dis- 

 integration must have been going on. It is probable that the 

 " clay-with-flints " covers a much more extensive area than would 

 appear from the map ; for in many places it is covered up beneath 

 thick accumulations of drift-deposits. It is a characteristic soil on 

 the top of chalk plateaux in nearly all parts of England. The flints 

 are frequently picked off the surface ; but it is doubtful if any real 

 benefit is attained by this means, as those underneath soon appear 

 at the surface. In some cases the mass of clay present in this soil 

 can only be explained by the presence of drift-deposits in addition ; 

 but, in many cases, we may look upon tbis flinty clay as the 

 insoluble residuum of a large thickness of Upper Chalk which 

 once existed there. 



The remaining soils covering up the Chalk district are chiefly 

 drift-deposits, which cover nearly the whole of the eastern side of 

 the county. The stiff boulder-clays of Essex extend over the 

 border, and give a general clayey character to a large portion of 

 the basins of the Stort, Ash, Rib, and Beane. This district, which 

 is described on the map as loamy clay, is much improved, both 

 mechanically and chemically, by the large amount of chalk present 



* ' Journ. Roy. Agric. Soc.,' vol. xvi, p. 170. 

 t Ih. vol. xii, p. 553, 



