32 



University of California. 



[Vol. 2. 



was wholly or partly excluded in the selection of material. Hence 

 it is reasonable to suppose that the feldspar, as a whole, is a little 

 more acid than the analysis would indicate. On the supposition 

 that the albite and anorthite molecules have the ratio Ab! An x it is 

 clear that less than half of the soda represented in the analysis of 

 the Point Sal rock can be contained in the feldspar. 



The pyroxene, as shown by the analysis, is a typical aluminous 

 basaltic augite. The small amount of alkalies is indicative of the 

 absence of the aegirine molecule. 



The composition of the analcite agrees very closely with the 

 average of the analyses of that mineral. The smaller areas through 

 the rock may differ from this to a limited extent. From the analyses 

 of the three important constituents it appears that, after deducting 

 the soda from the proportionate amount of feldspar, and considering 

 the whole of the alkalies in the augite as soda, there still remains 

 to be accounted for about three per cent of soda. This fact nega- 

 tives the view that all the analcite could have been leached from 

 the feldspars to fill original miarolitic cavities. The question then 

 of the origin of the analcite is considerably simplified. It can 

 have but one of two origins. It has either been introduced into 

 the rock after solidification, or it has resulted from the alteration of 

 some soda rich mineral, which filled the angular spaces between the 

 other constituents, the last portion of the magma to solidify. A 

 more detailed discussion of this question will follow later. 



Five grams of the rock were powdered, and a quantitative 

 separation made by means of Klein's solution and the magnet. 

 This gave the following results: Augite, 32.3; feldspar, 43.3; anal- 

 cite, 20; magnetite, 4; apatite, estimated, .004. The result, as far 

 as the feldspar and analcite are concerned, is only approximate. 

 Using the densities of the components, and their relative propor- 

 tions, the following table has been constructed. The percentage of 

 water in the different components, except the analcite, is deducted, 

 as the gravities given, particularly of the augite and the analcite, 

 are those of the purest material obtainable. The result obtained 

 agrees closely with the actual specific gravity of the rock. 



