LATER STAGES OF EVOLUTION OF IGNEOUS ROCKS 19 



the composition of all the liquids which can exist in equilibrium 

 with (are saturated with) crystals of forsterite. Similarly the field 

 of the pyroxenes contains the composition of all the Kquids which 

 can exist in equilibrium with (are saturated with) crystals of pyrox- 

 ene, the composition of the pyroxenes themselves being represented 

 by points lying along the conjugation line (dotted line joining 

 MgSiOj and diopside, between which there is an unbroken series 

 of pyroxene mix-crystals). Any liquid whose composition is 

 represented by a point lying on a boundary curve is in equilibrium 

 with (saturated with) two kinds of crystals, and any liquid whose 

 composition is represented by the point at which three boundary 

 curves meet is saturated with three kinds of crystals. There is only 

 one such liquid in the present system, that in equihbrium with 

 pyroxene, tridymite, and cristobalite. Fig. i is therefore a solu- 

 bility diagram. 



By drawing isotherms on the figure any point can be made to 

 represent (besides a definite composition) a definite temperature. 

 In Fig. I the direction of falling temperature is merely indicated by 

 arrows on the boundary curves, but in Fig. 2 the experimentally 

 determined isotherms or temperature contour-lines are shown. In 

 Fig. 2 any point within the forsterite field represents, not only the 

 composition of a liquid saturated with forsterite, but also the 

 temperature at which the liquid becomes just saturated with 

 forsterite and its crystallization begins. This is also, of course, 

 the temperature at which melting is complete, and Fig. 2 is there- 

 fore, besides a solubility diagram, a melting-point diagram. It is 

 more common to refer to such diagrams as melting-point diagrams, 

 emphasis being placed on the temperature aspect, but in compli- 

 cated systems it would, for many purposes, be better to emphasize 

 the composition aspect, solubility being the principal consideration, 

 as will appear later in the consideration of the crystallization of 

 magmas. 



Fig. 3 is analogous to Fig. i, showing the boundary curves 

 somewhat displaced in order to lessen the confusion of lines and 

 facilitate the discussion. Isotherms are omitted, but it must be 

 considered that any point represents both a definite composition, and 

 a definite temperature. 



