6 



wiscoxsi:n" acadeiiy scieis'ces, arts, and letters. 



to kaolinite is well shown by the following figures, which are per- 

 centages by weight, calculated on the original orthoclase: 



The last column corresponds to the composition already given, 

 for kaolinite, viz., silica, 43.3; alumina, 39.8; water, 13.9. The 

 change may also be conveniently indicated from the formulae as 

 folio ws : 



Orthoclase 



Removed - - _ 



Left 



Which corresponds to 



Added - - - 



Resulting Kaolinite 



6K„0+6A1..0, 

 6K„0+ 



-36SiO, 

 24SiO;; 



6AlA+12SiO„ 



'2^3 



AUOo2SiO., 



2H,0 



Al,0,,2SiO,+2H,0 



These calculations are made on the assumption that the alumina 

 is not removed. It appears however in some cases to be partially 

 removed.* The soluble substances resulting from this decomposi- 

 tion, the carbonates and silicates of potash and soda, and the bi- 

 carbonate of lime, pass off with the infiltrating waters, and reach- 

 ing the surface again, give rise to mineral springs, or add to the 

 solid contents of the drainage waters of the region. The felspar 

 may alter so as to produce certain zeolites when all of the protoxyd 

 bases are not removed,! and if the infiltrating waters carry mag- 

 nesia in traces a steatitic change may result. These are however 

 much rarer changes, and do not affect the object of the present 

 paper. 



Should the felspathic rock be contaminated with iron pyrites, 

 its decay may be much hastened.^ This may be in part due to 

 a direct action upon the silicate by the acid waters resulting from 



* Dana, loc. cit. 



t Bischof. Chem. Geol., p. 211 — Dana, loc. cit. 



j Dana's Mineralogy, p. 360. Geology of JSTew Jersey, p. 68 . 



