Fairbanks.] 



Geology of Point Sal. 



3' 



•containing analcite, the specific gravity was increased to 2.63. That 

 which remained at the bottom was drawn off, dried and carefully- 

 picked over with a needle to get rid of all grains to which augite 

 splinters were attached. The specific gravity of a number of the 

 grains was then taken, and it was found to range from 2.63 to 

 2.67. The optical properties of the feldspar would place it in the 

 basic portion of the labradorite series, and the anomalous specific 

 gravity can only be accounted for by the supposition that all the 

 feldspar is more or less affected by alteration. The analcite for 

 analysis was selected from the clear crystals filling the large cavities. 

 It is evident that this can not give a complete account of the rock, 

 for in addition to the feldspars partly changed to analcite, there 

 are other hydrated alteration products which make a reconstruction 

 of the rock in toto very diffiqult. Although it is impossible to 

 reach absolutely accurate results, those obtained can be considered 

 sufficiently near the truth to enable the question at issue to be 

 decided. The three following analyses, (I) of the augite, (II) 

 feldspar, (III) analcite, show the results obtained. No. IV, quoted 

 from Dana, is inserted for the purpose of giving the composition of 

 nepheline and showing its relation to analcite. 





I. 



II. 



III. 



IV. 



SiO, 



46.59 



52.72 



54-40 



44.08 



A1.A 



9.69 



30.46 



23.04 



33-28 



FeO 



4-75 









Fe 2 :) 



1.03 









CaO 



21.38 



1 1. 01 



.21 



1.85 



MgO 



13.89 









Na 2 

 K,0 



| 1-23 



37o 

 .42 



•3-33 

 .19 



16.00 

 4.76 



Ig- 



1.22 



1.44 



8.46 



•45 



Total 



99.78 



99-75 



99- 6 3 



100.12 



Sp. g. 



2-338 



263-267 



2.261 



2.6 



In the case of the feldspar, the result is very nearly what would 

 De expected from a study of the optical properties. It is probable 

 that the central core is a little more basic and that the outer rim 



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