OGDEH ca:^ok 397 



structure ; it differs in containing less qUartz, or, if present, in a more finely- 

 disseminated state, which may be sufficient ground for the slight difference 

 of physical habit. Still another variety, somewhat larger- grained, but with 

 the same constituents, is of particular interest, as, in the microscopic sections, 

 Professor Zirkel detected numerous minnte crystals of a dark-brown min- 

 eral, which he regarded as zircon, from its mineralogical habit, and from the 

 fact that he recognized the same microscopical forms in many of our 

 Archaean gneisses and schists, rocks in which zircon is frequently found, 

 but with us has never yet been observed macroscopically. In this rock, 

 the minute microscopical crystals, determined as zircon, were so abundant 

 that a portion of the small specimen collected was subjected, by Mr. R. W. 

 Woodward, to chemical analysis, for the purpose of detecting, if possible, 

 the presence of zirconia,-'-=a difficult problem where only a small amount 

 of the earth is present. He succeeded not only in detecting the presence 

 of zirconia, but in estimating the percentage contained in the rock The 

 analysis was made in duplicate, giving the following results : 



I. II. 



Zirconia...... 0.22 0.32 



As the ordinary methods employed for the separation of zirconia were 

 found not to be practicable in this case, Mr. Woodward devised a method 

 based upon the fact that zircon is but slightly acted upon by hydrofluoric 

 acid. The material supposed to contain zircon was powdered, and then 

 digested with hydrofluoric acid and a small quantity of concentrated sul- 

 phuric acid in a large platinum crudible until the rock itself was thoroughly 

 decomposed. After removal of the hydrofluoric acid by evaporation, the 

 bases, which had been in combination with the volatilized silica, were 

 brought into solution by protracted boiling with sulphuric acid and water, 

 zircon remaining as an insoluble residue. This residue being fused for a 

 long time with carbonate of soda, and the fused mass treated with water, 

 the soluble silicates thus formed were dissolved out, leaving a sandy zircon- 

 ate of soda insoluble in water, but soluble in hydrochloric acid. This is 

 simply a concentration of the zirconia from a large quantity of the rock. 

 The hydrochloric-acid solution gave all the characteristic reactions of 

 zirconia, with none of those by which it might be mistaken for other sub- 



