334 On the Structure of the Delta of the Ganges. 



the attractive affinities in operation, we as yet know nothing ; 

 and the only experimental fact connected with the subject 

 we possess, is that formerly alluded to. The great hard- 

 ness of the kankar may be explained from the peculiar 

 property bodies similarly constituted possess of forming 

 stony masses under water. The chemical composition of the 

 kankars both of India and England is identical with that 

 of the artificial mixtures which have been formed for the 

 purposes of hydraulic architecture. To exhibit this in one 

 instance, I annex the results of the analyses of Roman 

 cement and Ghazipoor kankar. 



Roman cement. Ghazipoor Tcanlcar. 



Carbonate of Lime, 



. 73.3 



... 72.0 



Silica, 



. 9.9 



... 15.2 



Alumina, ... 



. 4.4 



4.0 



Oxide of iron, 



. 11.3 



7.0 



Loss, 



. 1.1 



1.4 



Carbonate of magnesia, 



5} 



0.4 



100 100 



From these it will be seen how readily the induration of 

 the kankar may be explained from its close affinity to a sub- 

 stance of which the well known property is to harden imme- 

 diately on contact with water. 



Underlaying the bed of calcareous clay in which the kan- 

 kar first occurs, there is a thin bed of green siliceous clay, 

 extending from 60 to 65 feet in depth. The clay then 

 loses its colour, and continues to a depth of 75 feet, the 

 lower portion of it furnishing nodules of kankar. At 75 feet 

 a bed of variegated sandy or arenaceous clay commences, 

 and continues to the depth of 120 feet, occasionally traver- 

 sed by horizontal beds of kankar. Beneath this a stratum of 

 argillaceous marl 5 feet in thickness is found, and succeeding 

 it, there is a bed only 3 feet in thickness of loose friable 

 sandstones, the particles of sand being held loosely together 



