22 N. L. Bowen — Genetic Features of 



has gone farther the melilites have isotropic cores and 

 negative outer portions. A zoning of* this particular 

 nature has been noted in melilite from San Venanzo. 16 In 

 the natural rock ordinary olivine was not one of the 

 products of the reaction for the reason that the reacting 

 liquid differed from the experimental liquid, notably in 

 the potash content, with the result that mica was formed 

 instead. 



Space Relations of the Equilibrium Fields. 



The separation of melilite and olivine from liquids 

 whose total composition can be expressed as a mixture of 

 nephelite and diopside has already been discussed in 

 some detail and pictured in fig. 2. As an aid to the under- 

 standing of equilibrium relations in the rock we have 

 described, it is perhaps desirable to present the results 

 in a somewhat different manner, that is, in terms of a 

 composition tetrahedron having as its base the CaO- 

 MgO-Si0 2 triangle and nephelite at the other corner. 

 The compounds and their equilibria with liquids for all 

 compositions lying on the base have been studied by 

 Ferguson and Merwin, and our present purpose is to 

 describe how their fields are affected as we pass out into 

 the tetrahedron, that is, as nephelite is added. It is 

 apparent from the equations that have been given above 

 that even four components are not sufficient to express 

 accurately the composition of all the phases present but 

 we could, as in the pseudo-binary diagram (fig. 2), mark 

 the tetrahedron out into fields showing the nature of the 

 solid phase in equilibrium with liquid, the total com- 

 position of the mixture being given by a point in the 

 tetrahedron. 



No attempt has been made to map out the fields 

 throughout the whole tetrahedron, which would be a labor 

 of many years. Attention has been concentrated upon 

 the position of the melilite, olivine and pyroxene fields as 

 having particular interest in the present connection. 



The fields that have been delimited for the MgO-CaO- 

 Si0 2 mixtures constituting the base will pass upward 

 into the tetrahedron, the boundary curves becoming 

 boundary surfaces. The position of these surfaces as 



16 Kosenbusch, Mikroskopische Physiographie, I. 2, p. 71, 1905. 



