174 Mr. Webster on the Strata lybtg over the Chalk. 



This chalk marl is never quite so white as chalk, having generally 

 a tinge of yellow, and sometimes of grey and brown. It also con- 

 tains nodules and beds of a more indurated marl, which is usually 

 called the grey chalk from its dark colour, which varies from a 

 light to a dark grey and brownish grey. Like all argillaceous lime- 

 stones it possesses in a considerable degree the property of setting 

 under water when calcined and made into mortar ; and it has been 

 used with great success for this purpose in building the London 

 Docks. The part easily reducible to the pulverulent state by the 

 moisture and frost is a most valuable manure when employed judi- 

 ciously in certain soils. This stratum contains no flints. 



The middle and upper strata consist of chalk of extreme white- 

 ness and purity, and are chiefly distinguished from each other by the 

 upper one containing layers of flint nodules which do not occur in 

 the lower. The chalk without Jii?its is most frequently somewhat 

 harder than that ivith Jlints^ and hence they are sometimes distin- 

 guished as the hard and soft chalk ; but from some observations 

 which I have made in Dorsetshire, it will appear that the hardness, 

 or degree of induration, does not always mark a particular bed, 

 the flint chalk being in some places much harder than that without 

 flints in others. 



On the subject of the nodules and lamiuGe of flint in the upper 

 chalk, the observations of Sir Henry Englefield have thrown great 

 light, and will be mentioned in his intended work on the Isle of 

 Wight. 



The several beds of these chalk strata, which vary in thickness 

 from a few inches to several feet, are frequently separated from each 

 other not only by layers of flint nodules but frequently also by a 

 marl containing a considerable proportion of clay, and this substance 

 also sometimes fills up the diagonal fissures which cross the strata. 



