142 MK. A. J. JUKES-BKOWNE ON [May I906, 



4*33 grammes ; this is equivalent to 34-64 grammes per cubic inch. 

 Another half-inch cube, from the clay in a pipe at the same place, 

 weighed 4*5 grammes, equivalent to 36 per cubic inch. From two 

 samples obtained by Mr. J. Saunders near Luton, two good cubic 

 inch-blocks were cut, one weighing 33 and the other 33*53 grammes. 

 From these figures we deduce a mean value which cannot be far 

 wrong, thus one cubic inch of 



Grammes. 



Knebwortk clay weighs 34-64 



Do. do. (pipe) „ 36-00 



Luton (No. 1) clay „ 33*00 



Do. (No. 2) do „ 33-53 



4) 137-17 = average 34-04. 



An average cubic inch of Clay-with-Flints from these places 

 weighs 34 grammes, which is -068 of a pound : consequently, a cubic 

 foot of clay weighs 1728 x '068 = 1175 lbs. 



Thus, if the Clay-with-Flints has been derived from the Chalk, 

 its weight as compared with that of Upper Chalk is as 117*5 to 98. 

 Further, around the London Basin the clay must have been chiefly 

 obtained from the solution of the zones of M arsupites and Micraster 

 cor-anguinum, for it is on one or the other of these that the Reading 

 Beds generally rest. If, therefore, we suppose that 100 cubic feet of 

 chalk be dissolved, half consisting of one zone and half of the other, 

 and assuming the percentage of insoluble residue to be 1*24 by 

 weight, the mass of the residue will be 117*5 : 98 : : 124 : cc. This 

 works out as 1*03 ; consequently, the solution of a column of 200 

 vertical cubic feet of these two zones will yield 2*06 cubic feet of 

 clay. In other words, in order to produce a layer of clay 2 feet 

 thick, a thickness of 200 feet of such chalk will have to be dissolved, 

 and the solution of 100 feet will produce only a 1-foot layer. If com- 

 posed entirely of chalk of the Micraster cor-anguinum-zone, the layer 

 produced by the solution of 100 feet will be 1*2 feet thick. 



In the area between the London and Hampshire Basins, it is 

 mainly the chalk of the zones of Actinocamax quadratus and Mar- 

 supites which would have suffered, and these only yield a combined 

 percentage of *88 by weight ; the percentage by mass will be only *73. 

 Consequently 100 cubic feet from these zones will produce a layer 

 of clay only 9 inches thick, and it would take 200 feet of them to 

 produce a layer of 18 inches over the whole area, supposing it to be 

 all retained on the area and none of it lost by water-transport 

 during the process of solution. It is very unlikely, however, that 

 it could be so retained on the plateaux without any waste, and 

 much more probable that half of it would be carried down into 

 the valleys. 



In any case, it is clear that, if Clay-with-Flints consists half of 

 clay and half of flints, the possibility of the derivation of the clay- 

 portion from the Chalk must depend entirely upon the thickness of 

 Chalk which can be proved to have been destroyed in any given 

 district. 



