24 JOURNAL OF GEO LOGY SUPPLEMENT 



Crystallization with zoning. — -It has been stated that crystalHza- 

 tion takes place after the manner described for the foregoing 

 mixtures when perfect equihbrium is obtained. The early crystals 

 of forsterite must be partly or completely redissolved and the 

 crystals of pyroxene must, by diffusion in the solid, continually 

 change composition throughout their entire mass. This result 

 could be accomplished only with extremely slow cooling and with 

 continuous mixing to prevent the sinking of crystals. With quicker 

 cooling an entirely different result will be obtained. 



If the cooKng is at such a rate that forsterite crystals are not 

 redissolved and pyroxene crystals once separated do not change 

 their composition, then from any of the liquids M, P, or D of Fig. 3 

 forsterite would separate first as before until the temperature of the 

 point K was reached, when the liquid has the composition K. At 

 this temperature pyroxene of composition L begins to separate as be- 

 fore, but re-solution of forsterite does not take place in this case. The 

 composition of the liquid therefore does not follow the boundary 

 curve but crosses the pyroxene field and meets the boundary 

 curve pyroxene-tridymite at a point lower than E, say G. In the 

 meantime the composition of the pyroxene separating has changed 

 from L to T, and the pyroxene crystals therefore show zones of 

 continuous change of composition varying from L to T. When 

 the temperature has reached that of the point G and the composition 

 of the liquid is G, tridymite begins to crystallize and the com- 

 position of the liquid changes along the boundary curve pyroxene- 

 tridymite. Meantime the composition of the pyroxene separating 

 changes from T toward pure diopside, and final crystallization takes 

 place only when the temperature is that of the eutectic diopside- 

 tridymite, when the remaining infinitesimal amount of liquid has 

 the composition of this eutectic and the crystalline phases separating 

 are tridymite and pure diopside. 



Any of the liquids M, P, or D will, then, if crystallized in this 

 manner, consist of forsterite, tridymite, and pyroxene varying in 

 composition from L to pure diopside. The actual amount of 

 pyroxene approaching L in composition is relatively large; the 

 amount approaching diopside, relatively small; the amount of 

 pure diopside, infinitesimal. The kind of crystallization which 



