334 



DE. F. DIXET ON THE 



[vol. Ixxviii, 



Aa, Bb, Cc to represent temperatures, the points a, b, c being the 

 freezing-points of the minerals A, B, C. Through a, b, and c will 

 pass freezing-point surfaces for mixtures of A, B, and C, 

 respectively, with the remaining two minerals.^ Each pair of 

 surfaces will intersect in a line formed by the eutectic points of 

 the pair of minerals at different temperatures ; the three lines 

 representing the binary eutectics will meet in a point representing 

 the ternary eutectic. Let Ej^E, E^E, and E^E, represent the 

 projection of the three lines on the area ABC; then the inter- 

 section (E) of these lines is the required point. 



The order of events in the cooling and crystallization of a 

 magma of comjoosition P will be as follows : an indicating point 

 vertically above P will descend as the magma cools, until it reaches 



SiO 



the freezing-point surface of the felspar (A) at a ; then the felspar 

 will begin to crystallize out, and the indicating point will travel 

 along the surface on a line (the projection of which is PQ) directly 

 away from a, until it meets the felspar-magnetite eutectic line at a 

 point projected at Q. The magnetite will then crystallize in 

 eutectic proportions with the felspar, and the indicating point will 

 travel down the binary eutectic line of these two minerals until it 

 reaches the ternary eutectic point, which is projected to E. Then 



1 See A. Harker, ' The Natural History of Igneous Eocks ' 1909, %. 60, 

 p. 202. The notation which I have adopted is after that employed by him. 



