AND LOAMS OF WEST HERTS. 1J7 



the clay is intimately associated with the deposits of loam, as 

 is clearly seen in Section 3, showing the beds east of Wigginton. 

 Its thickness is very variable. 



The question of the origin of the clay-with-ilints has been 

 a cause of much speculative reasoning. The general belief is that 

 the deposit is largely tlie result of the solution of a portion of 

 the Upper Chalk by rain or other waters containing carbonic and 

 other acids. This explanation, put forward by Sir C. Lyell, 

 iSir Joseph Prestwich, and Mr. W. Whitaker, appears to be the 

 best yet offered. Several reasons may be adduced in support of 

 the theory, but want of space does not allow of these being 

 discussed. When moist, the clay is plastic or adhesive like other 

 clays, but there is a peculiarity about the normal clay-with-flints ; 

 its adhesiveness is of an unpleasant, greasy nature, such as might 

 be due to the presence of gelatinous silica. Now when chalk is 

 dissolved by means of very dilute hydrochloric or other acid, 

 a fawn-coloured residue is left, and this, after being washed, has 

 an unpleasant, greasy adhesiveness, very similar to that possessed 

 by a normal specimen of clay-with-flints. By incorporating small 

 quantities of oxide of iron with such a residue, an artificial 

 "clay," undistinguishable from a normal sample of clay-with- 

 flints, may be prepared. This shows that dissolution of the chalk 

 is a possible cause. 



Before closing this partial description of the clay-with-flints, an 

 account of the mineral composition of two samples of the clay may 

 be of interest. The weights of the constituents are given in 

 grammes. A striking feature about the Bovingdon sample is the 

 presence of a comparatively large amount of muscovite or potash- 

 mica, there being numerous glistening white plates of that mineral. 



Mineral Cojiposition of Clay-with-Flints. 



Locality. Impalpable clayey matrix. Quartz. Flint. Muscovite. 

 Bovingdon 1-73 0-43 1-3 0-04 



Felden 0-88 0-12 0-48 Traces. 



LoAMi OR BeiCK-EARTH. 



Extensive deposits of loam occur over many parts of the area 

 where the clay-with-flints is found, and the association of the two 

 deposits is sometimes so intimate that it is difficult to decide where 

 one deposit begins and the other ends. The loam is essentially 

 a mixture of sand and clay in various proportions, but it often 

 contains flint-pebbles, blocks of the well-known Hertfordshire 

 pudding-stone, and unworn flints which are sometimes very large, 

 as in the beds at Champneys, Leverstock Green, and Berkhamsted 

 Common. The flint -pebbles are often well rounded, and must 

 have been derived from Tertiary beds. 



The beds of loam vary greatly in tliickness, there being only thin 

 cappings in some places, while in others the loam is thirty or more 

 feet thick. At Leverstock Green, Woodlane End, Bennett's End, 

 Champneys, Berkhamsted Common, Bernard's Heath, and some 



