S4 DR THOMAS STEEEY HUNT ON THE 



scaiiolite and epidote groups (§ 81)."' The greater stability of those which belong to the 

 more condensed types is shown in their superior resistance to decay, and is thus of geolo- 

 gical signifiance. The relations of anhydrous to hydrous species of aluminous doixble sili- 

 cates appear to be of less importance, when we consider what secondary causes will deter- 

 mine the formation either of a hydrous or an anhydrous species, of a zeolite or a feldspar."" 

 The relations of the bases, potash, soda, and lime to each other, and to magnesia and other 

 protoxyd-bases, are next to be considered, alike for the double aluminous silicates and for 

 simple silicates of protoxyds. 



A system of classification, constructed in accordance with these principles, has 

 already been indicated in the preceding illustrations of the crenitic hypothesis, and will, 

 it is believed, be found of fundamental importance for the student of mineral physiology ; 

 since it is based on the genetic processes by which the species of the mineral world have 

 in most cases been formed. The iwinciples which it embodies, will be found not less 

 applicable to compounds of igneous origin than to those formed by aqueous processes. 



§ 109. In considering the origin of crystalline stratified rocks formed, in accordance with 

 our hypothesis, in all cases with the concurrence of water, questions connected with the 

 process of crystallization of mineral species, and of their condition when first deposited, are 

 of much importance. The most familiar case is that of the direct separation of matters in a 

 crystalline condition, as happens from the evaporation or the change of temperature of the 

 solvent, or from the generation of new and less soluble compounds, as in many cases of che- 

 mical precipitation. In this connection, it should be noted " that many such compounds, 

 when first generated by double decomposition in watery solutions, remain dissolved for a 



greater or less length of time before separating in an insoluble condition There 



is reason to believe that silicates of insoluble bases may assume a similar state, and 

 it will probably be shown one day that for the greater number of those oxygenized com- 

 pounds, which we call insoluble, there exists a modification soluble in water. In this con- 

 nection also may be recalled the great solubility in water of silicic, titanic, stannic, ferric, 

 aluminic and chromic oxyds, when in what Graham has called the colloidal state." "' In 

 writing the above, in 18*74, reference was also made to my own earlier observations on the 

 solubility, under certain conditions, of carbonate of lime, which are subjoined. 



§ 110. " The recent precipitate produced by a solution of carbonate of soda in chlorid 

 of calcium is readily soluble in an excess of the latter salt, or in a solution of sulphate of 

 magnesia. The transparent, almost gelatinous magma, which resixlts when solutions of 

 carbonate of soda and chlorid of calcium are first mingled, is immediately dissolved by a 

 solution of sulphate of magnesia, and by operating with sokitions of known strength 

 [titrated solutions] it is easy to obtain transparent liquids holding in a litre, besides three 

 01 foixr hundredths of hydrated sulphate of magnesia, 0.80 gramme, and even 1.20 grammes, 

 of carbonate of lime, together with 1.00 gramme of carbonate of magnesia; the only 

 other substance present in the water being the chlorid of sodium equivalent to these car- 



"^ See, in this connection, the author On the Objects and Method of Mineralogy, Chem. and Geol. Essays, pp. 

 4.52-458 ; also the same, pp. 445-447. 



* On the relations of hydrous to anhydrous species, see farther the author in Trans. Roy. Soc. Can., vol. i., 

 part 4, p. 208. 



"' Hunt, Chem. and Geol. Essays, p. 223. 



