[ xvii ] 
Since it has not been ufual to confider the 
earths and ftones as the fame, in regard to the 
principles of which they confift, and only diffe- 
rent from one another by the greater or leffer 
hardnefs and coherency of their particles^ I hope 
for excufe, in not being able to profecute this 
Elfay fo far as to point out a particular earth 
for each kind of ftone. Perhaps alfo fome of 
the mineral bodies are already fo much harden- 
ed, that no earths of this kind are to be found ; 
or, perhaps, the little knowledge we have of 
them is owing to the negledl of not collecting 
earths with the fame earneftnefs as we do ftones 
and ores. If, therefore, all the earths which 
are ranged in certain modern fyftems, and 
there diftinguiftied from one another, in regard 
to the difference of their colours and places 
where they are found, had fallen into the hands 
of Mr. Pott, and been tried by him in the fire* 
as he has tried the ftones, and been deferibed in 
his Llthogeognejia^ it is very probable that we 
now fhould know thefe bodies better, and their 
number would certainly be lefs perplexing. 
The hints which are here given may, how- 
ever, tend to promote the intended point for 
the future ; and then perhaps the earths will be 
found not to be quite of fo many different kinds 
as I have here been obliged to divide them into, 
for want of perfedly knowing their affinities and 
their feveral origins; becaufe we haveftrong rea- 
fons to believe that the calcareous and argillaceous 
earths are the two principal ones, of which all 
b the 
