368 T. W. E. DAVID, F. B. GUTHRIE, AND W. G. WOOLNOUGH. 



contains deposited silica from the decomposition of the nepheline, 

 was treated with 30 cc. of 10 per cent, solution of NaHO. It 

 was boiled for about two minutes, diluted, and filtered through a 

 weighed filter-paper. The residue dried at 100° C. and weighed, 

 gives the amount unattacked by hydrochloric acid. The alkaline 

 filtrate was acidified, evaporated to dryness and the silica deter- 

 mined in the usual way. We thus obtain the proportion of silica 

 separated out when the rock was attacked by HC1 as well as the 

 proportion which went into solution in the acid. These were 

 added together and entered in the analysis as Si0 2 decomposable 

 by HC1. If stated separately they are as follows: — 



I. III. IV. 



Separated Si0 2 ... 46-73 46-88 46-91 



Dissolved Si0 2 ... 5 11 7-02 6-25 



51-84 53-90 53-16 



With regard to the distribution of the different minerals in the 

 rock, Mr. Card has kindly given us the results of his calculation. 

 He finds the following to represent roughly the percentage mineral 

 constitution of the rock according to the analytical data, taking 

 as a basis the mean of the three analyses : — 



Felspar (orthoclase and albite = soda-orthoclase) ... 45 -4 



Nepheline 33-6 



JEgirine-augite ... ... ... ... ... 21*0 



1000 



The assumptions made are the following: — 



1. The whole of the Fe 2 3 is contained in segirine. 



2. The FeO and MgO are contained in a pyroxene together 



with the CaO. 



3. A MnO Si0 2 molecule is present. 



The whole forming an segirine-augite of the composition — 



(Na 2 0. Fe 2 3 . 4 Si0 2 . 

 i (FeO. MgO) CaO. 2 Si0 2 . 

 ( MnO. Si0 2 



4. The proportion of K 2 to Na 2 in the nepheline is 1 : 5. 



