The Genesis of Clay Stones. — Nichols. 327 
Possibly the presence of finely divided lime-bearing minerals, 
the basic feldspars for example, in the rock flour of glacial 
clays may also provide some of the lime. 
Shells, such as are found in the clays, contain their calcium 
carbonate in two forms, as arragonite and calcite.* 
As arragonite is more soluble than calcite,f it follows from 
the properties of saturated solutions that the arragonite must 
disappear and its material be deposited as calcite upon any 
already existing calcite particles. 
The principles upon which this change depends are well and 
clearly expressed by Ostwald;]; as follows : 
" The equilibrium between the solution and the solid has its 
origin in their mutual action, and is therefore dependent upon 
the nature of both. A definite solution pressure may be as- 
cribed to every solid with respect to any given liquid at a 
given temperature. Solution will go on until the opposing 
pressure becomes equal to the solution pressure. Consequently 
tha solubility of a chemical compound is determined only 
when it is stated what definite solid is referred to.'' 
§" The phenomena of supersaturated solutions are now com- 
prehensible. If a salt solution is allowed to cool or evaporate 
there is, in general, no reason for the deposition of a solid. 
If, however, the osmotic pressure in the solution exceeds the 
pressure of one of the salts, which may separate, and a parti- 
cle of this salt is brought into contact with the solution, a 
rapid separation of the salt results and continues until a state 
of equilibrium is reached. Such solutions are called super- 
saturated, but it is clear that the supersaturation is only 
with respect to a certain definite solid: without this reference, 
the expression is meaningless." 
"From a cold solution of sodium sulphate, there can sepa- 
rate NagSO^ 4- 7 HgO as well as NagSO^ + 10 \if>. As the for- 
mer is more soluble than the latter, solutions can be prepared 
which will crystallize in contact with Na.^ SO^ -|- 10 HO, i. e., 
are supersaturated with respect to this salt, while on the other 
*Sorby.- Quar. Jour. Geol. Soc. 1879, p. 61: quoted by Geikie: Text 
Book, p. 78. 
fGeikie: Text Book of Geology, p. 78. 
^Outlines of Theoretical Chemistry, p. 149. 
§Ibid, p. 151. 
