On the Structure of the Delta of the Ganges. 333 



sand (silica) and clay (alumina) reduced to impalpable pow- 

 der be made, by suspending the two substances in the same 

 fluid, and if part of the fluid be evaporated till a plastic com- 

 pound result, the silica of the compound will after it has 

 stood for a short time undisturbed segregate into small hard 

 nodules similar exactly to those of flint in chalk.* On com- 

 paring the circumstances of this experiment with those 

 under which the class of bodies to which kankar belongs 

 are formed, a striking and interesting similarity will be ob- 

 served. Thus we know the chalk to be an aqueous deposit, 

 and there must have been a period at which its consistence 

 would be plastic as the above, it seems therefore to be 

 no unwarrantable inference that those segregations of 

 the particles of silica disseminated throughout the mass 

 which now appear as flints, took place during this time. 

 Similarly, and more closely analogous to the kankar, the 

 London clay must also at one period have been in a plastic 

 state, and as by analysis it is well known that calcareous mat- 

 ter is largely disseminated throughout its mass, it is probable 

 that the nodular Septaria were then formed by the segrega- 

 tion of their constituents. It is only necessary to allude to 

 the other analogous cases in the strata of the Paris basin, 

 where the nodules are not confined to flint and limestone 

 alone, but are of gypsum, hornstone, celestine, &c. all of 

 whose associated strata are known to be of aqueous origin. 

 These therefore furnish the grounds on which I have been 

 led to consider the kankar as formed by the segregation of 

 the particles of calcareous matter, disseminated throughout 

 the body of the clay with which it is associated, and to be 

 nearly contemporaneous in its origin with this. Of the pe- 

 culiar circumstances which dispose substances to this se- 

 gregation, of the proportion which the segregating material 

 should bear to the general mass, or of the exact nature of 



* Babbage. Economy of Machinery. 



