182 



PERIDOTITE. 



chromites next in order contain 9.80, 16.80, three between 17 and 18 per cent, 

 then 21.16 and 31.20. The chrompicotite, however, contains 56.54 per cent 

 of chromic oxide. The highest percentage is 77.00, in a doubtful analysis 

 by C. H. Pfaff; while the next lower has 64.17 per cent. 



The highest percentages of alumina in the chromites are 30.17, 27.83, and 

 24.71, but in general it diminishes in amount as the chromium increases. 

 In picotite the alumina is high, being 50.34, 52,47, 53.93, 55.34, and 56.00 

 per cent, and in this occurs the only real chemical difference between pico- 

 tite and chromite. In the chrompicotite the alumina is 12.13. The percent- 

 age of magnesia is about the same in both tlie chromites and picotites, and 

 does not bear any observiible proportion to any other element. The highest 

 percentages are, in picotite 23.59, and in chromite 28.71 and 25.40. 



The oxides of iron irregularly increase as the chromic oxide does, rising 

 as high as 45.22, 48.46, and 62.02, while the lowest percentages are 2.30, 

 5.60, and 9.00; but picotite contains the following: 22.27, 21.42, 15.25, 

 24.60, and 24.90, and the chrompicotite 18.01. The silica and lime diminish 

 irregularly as the chromic oxide increases. The liighest percentages of silica 

 are 26.01, 26.70, and 14.211 ; and of lime 24.36, 13.26, and 10.55. 



The minor and rare elements are the oxides of nickel, manganese, and 

 carbon. 



The more general relations are perhaps best shown in the tables inserted 

 here in the text. 



TABLE I. 



Table I. has placed in its upper line the number of analyses in each set ; 

 in the second line, the jjercentage limits of the chromic oxide in these 

 analyses ; while on the third line is placed the percentage limits of the 

 magnesium, aluminum, ferric, ferrous, silicon, and calcium oxides. Below are 

 given the number of analyses found in the above limits. Thus, for instance. 



