GEOLOGY AND PETROGRAPHY OF THE PROSPECT INTRUSION. 513 



Table of Norms. 





I. 





II. 



III. 



(Specimen 



D 





(Specimen B) 



Orthoclase 



3.34 





6.12 



7.23 



Albite 



17.29 





22.53 



20.44 



Anorthite 



20.85 





27.52 



18.07 



Nepheline 



1.89 





2.00 



3.98 



Diopside 



26.51 





13.87 



20.15 



Olivine 



7.66 





8.95 



19.50 



Magnetite 



9.28 





7.89 



3.48 



Ilmenite 



8.36 





4.71 



3.34 



Apatite 



0.34 





0.67 



1.01 



15. Variations of Composition in the Main Mass. 

 A feature of considerable interest in the Prospect mass 

 is the changes in the relative proportions of the mineral 

 constituents from point to point, because they are evidence 

 of difference in situation whilst the magma was cooling. 

 The probable nature of the processes of differentiation 

 which operated will be discussed in the last part of this 

 paper, and in this section we confine ourselves to a state- 

 ment of the facts. The quantitative distribution of the 

 minerals was ascertained both by comparison of a number 

 of specimens, whose mineral 'compositions were determined 

 by calculation from analyses and by measurement, and by 

 microscopic study of sections of several other specimens 

 which did not seem sufficiently individual in character to 

 require measurement. The evidence of the former kind is 

 contained in the following table, in which are assembled the 

 mineral compositions of all the specimens which have been 

 determined quantitatively. It will be noticed that one 

 specimen is here included (see column V) which has not 

 been in our hands for description, and of which the precise 

 locality is unfortunately unknown. Its mineral composition 

 was calculated from the analysis published by the Geological 



