REVIEWS JO 3 



micro-porphyritic structure, and ophitic spinel-basalt, a spinel-bearing 

 feldspathic basalt of micro-porphyritic and divergent-radial structure, 

 a corundum-bearing nepheline-basalt, melilite-basalt bearing spinel, 

 corundum-nephelinite, and coarsely trachytic corundum-bearing anor- 

 thite-nepheline mixtures. 



Corundum and spinel have frequently been obtained synthetically 

 by both "wet" and dry methods, and an examination of the literature, 

 no less than the casual production of these minerals in preliminary 

 experiments, showed that an excess of alumina readily induces the crys- 

 tallization of free A1 2 3 , in the form of corundum, and with relatively 

 high magnesia and iron in addition, produces spinel. The minerals 

 were isolated and analyzed ; both green and black varieties of spinel 

 were obtained, the one chlorospinel, the others pleonast and hercy- 

 nite. A comparison of the magma analyses with the relative amounts 

 of these minerals produced, shows that alumina plays the principal role 

 in the production of spinel as well as corundum. On the hypothesis 

 that the crystallization of free alumina indicates supersaturation, it was 

 believed that precise saturation, or the condition of the magma after 

 the excess of A1 2 3 had crystallized out, should give a ratio of alumina 

 to the bases of i : i, that being a constant in most of the alumino- 

 silicates (feldspar, nepheline, haiiyne, sodalite, mica, etc.). This was 

 confirmed by eight analyses of the glass from which the corundum and 

 spinel had crystallized ; these gave the ratios 



(K a O-Na 2 0-CaO):Al 2 8 :Si0 2 =(ib) 0.9:1:1.9; (2b) 0.9:1:2.3; 

 (3b) 0.9:1:2.3; (4b) 0.9:1:2.3; (5b) 0.9:1:2.3; (6b) 1.2: 1:1.9; (7b) 

 1.1: 1:3.4; (8b) 1:1:3.2. 



Thus with variable silica,the ratio of A1 2 3 to the bases averaged = 1. 

 To confirm this result a special series of test mixtures were melted 

 up and crystallized. These tests, made variously with magmas of the 

 composition of basic and acid feldspars, with the alkalies and silica in 

 varying proportions, and under varying conditions of cooling, gave the 

 following important results : 



1. A silicate magma is saturated with alumina, when the ratio of 

 the bases to alumina is equal to 1. 



2. Saturated aluminosilicate magmas of mixed composition and of 

 varying silica contents, are capable at high temperatures of dissolving 

 alumina and forming supersaturated solutions. 



3. Pure soda-aluminosilicate magmas dissolve alumina in large 



