ERUPTIVE ROCKS. 



591 



T.iBLE III. — Determinations of lead, sine, cobalt, and barium, chiejii/ in errptlve rocks. 



N. B. — The blank spaces under the he 

 resalta are given, no tests were made. 



Complete analysis in Table L 



Dgs PbO, etc., do not indicate absence of the respective oxides. Wbe 



KEMARKS ON TABLES I, II, AND III. 



Iii,soluble silicates were decomposetl by fusing with alkaliue caibouates for the 

 tletermiiiatiou of silica, titauie acitl, and all bases except the alkalies. 



Ferric oxide and alumina were separated either by pure potassium hydrate, or 

 more generally by ammonium sulphide, after addition of tartaric acid and ammonia. 



For the determination of ferrous oxide, treatment with sulphuric acid in sealed 

 tubes at about 200° C. was employed in cases where complete decomposition of the 

 silicates could thus be effected, and the solution titrated with potassium perinangauato. 

 For the decomposition of refractory silicates, pure hydrofluoric acid, distilled from 

 a platinum retort, was employed, the solution being effected in platinum vessels with 

 careful exclusion of air. The iron was then determined as above. 



