Geology.] 
NA.TU1UL HISTORY. 
597 
the former, and while the deposition of similar calcareous mat" 
ter was. still in progress, the portions first consolidated must 
have been shattered by considerable violence. But, where 
no such fragments exist, the unequal diffusion of components 
at first uniformly mixed, — and even the formation of nodules 
differing in proportions from the paste which surrounds 
them, may perhaps admit of explanation, by some process 
analogous to what takes place in the preparation of the com- 
pound of which the ordinary earthenware is manufactured; — 
where, though the ingredients are divided by mechanical 
attrition only, a sort of chemical action produces, under cer- 
tain circumstances, a new arrangement of the parts*. And 
this explanation may, probably, be extended to those no- 
dular concretions, generally considered as contemporaneous 
with the paste in which they are enveloped, the distinction 
of which, from conglomerates of mechanical origin, forms, in 
many cases, a difficulty in geology. What the degree may 
be, of subdivision required to dispose the particles to act thus 
upon each other, or of fluidity to admit of their action, 
remains still to be determined. 
* The clay and pulverized flints are combined for the use of the 
potter, by being first separately diffused in water to the consistence 
of thick cream, and when mixed in due proportion are reduced to a 
proper consistence by evaporation. During this process, if the 
evaporation be not rapid and immediate, or if the ingredients are 
left to act on each other, even for twenty-four hours, the flinty par- 
.ticles unite into sandy grains, and the mass becomes unfit for the 
purposes of the manufacturer. I am indebted for this interesting 
fact, which, I believe, is well known in some of the potteries, to my 
friend Mr. Arthur Aikin. And Mr. Herschel informs me, that a 
similar change takes place in recently precipitated carbonate of 
copper ; which, if left long moist, concretes into hard gritty grains, 
of a greet! colour, much more difficultly soluble in ammonia than 
the original precipitate. 
