F. T. 8. Houghton— Olivine Q 'abb ro from Cornwall. 505 





I. 



II. 



III. 



Water 



.. 4-38 ... 



3-96 ... 



830 



Silica 



.. 45-73 ... 



... 44-81 ... 



... 41-13 



Alumina ... . 



.. 22-10 ... 



21-62 ... 



13-56 



Ferric Oxide 



.. 0-71 ... 



0-86 ... 



219 



Ferrous Oxide 



3-51 



3-73 ... 



6-19 



Lime 



.. 9-26 ... 



9-19 ... 



672 



Magnesia ... . 



.. 11-46 ... 



... 11-78 ... 



... 22-52 



Potash ... . 



.. 0-34 ... 



0-35 ... 



0-83* 



Soda 



.. 2-54 ... 



2-30 ... 



0-96 



Eesidue ... . 



.. 



3-17 ... 







100-03 101-77 102-40 



* This proportion of Potash and Soda hardly agrees with that given in the 

 analysis of the felspar (2). 



From a comparison of these analyses, the two rocks would appear, 

 at first sight, to be of very different natures; but on examining the 

 results, it is seen that, whereas the Cornish rock contains only 33 per 

 cent, of olivine, the Volpersdorf has 45 per cent. ; a difference in 

 proportion quite observable under the microscope. 



There being this great difference in the proportion of the minerals, 

 it remained to determine the composition of the felspar. A sufficient 

 quantity, freed as much as possible from olivine, was submitted to 

 analysis with the result detailed in No. I. No. II. is an analysis of the 

 felspar of the Volpersdorf rock quoted by Zirkel (loc. cit.). 



I. II. 



Water 3-19 1-87 



Silica 49-65 47-05 



Alumina 29-35 30-44 



Iron Oxides ... 0-59 1 56 



Lime 12-18 16-53 



Magnesia 0-46 0-09 



Potash 0-48 0-78 



Soda 361 2-10 



99-51 100-42 



Oxygen ratios 1:3:5-7 1:2-7:4-5 



This gives a composition for the felspar of the Cornish rock of 

 11 {A1 2 3 . CaO. (Si0 2 ) 2 } + 3 {A1 2 3 . Na 2 0. (Si0 3 ) 6 } 

 Anorthite Albite 



which is an excess of two molecules of anorthite over that necessary 

 to form labradorite. The felspar thus proves to be — like that of 

 the Volpersdorf rock— not a true anorthite, but possibly a_ decom- 

 posed labradorite, which has lost some of its alkalies ; this being 

 suggested by a slight excess of silicate of alumina over and above 

 that required by the formula above given. Vom Rath suggests a 

 similar origin for the felspar of the German Forellenstein. We 

 may, therefore, without hesitation, place the Cornish rock among the 

 Forellensteins— adding another locality to the few already known 

 for this rock. 



