THE ISOMORPHOUS RELATION 



167 



eutectic pair. The first crystals to appear are of mixed composition^ 

 somewhat richer {cd in the figure) in the higher melting component (A) 

 than the original composition, and the remaining solution is in conse- 

 quence left somewhat poorer in this component. These first crystals will 

 be followed by others containing less and less of A than cd, until the mix- 

 ture finally ends its crystallization with mixed crystals of a composition 

 well over toward B. In the figure, the lower curve represents the com- 

 position of the crystals which form, and the corresponding points (found 

 by drawing horizontal lines through the points desired) of the upper 

 curve, that of the solution left behind. It is a process in which the com- 

 positions change continuously throughout, and obviously the heat distri- 

 bution also. There is, therefore, no sharp melting or solidifying point. 

 Whether the resulting crystals will 

 vary continuously in composition 

 from c to e, or will appear in broad 

 or narrow bands with insignificant 

 transition stages, or will even come 

 out homogeneous and of composition 

 /, will depend in part upon the char- 

 acteristic properties of the ingre- 

 dients and the nature and extent of 

 the disturbances to which the cool- 

 ing system is subjected. All these 

 cases are fairly common in igneous 

 rocks. It should be borne in mind, 

 however, that if the solution is of a 

 character to follow promptly all 

 changes of pressure and tempera- 

 ture as they occur, no bands will 

 appear. The appearance of bands is direct evidence of slow transitions 

 and disturbed conditions of cooling; their common occurrence in the 

 rocks is. indicative of frequent and considerable interruptions during the 

 cooling process. 



If under-cooling occurs, no crystals separate when the solution reaches 

 the temperature.??, but cooling continues along the line hf for a greater 

 or smaller interval, as the case may be. If it were to begin crystallizing 

 at m. the process would be exactly the same as before, except that tlie 

 range of possible compositions would be smaller — that is, there would be 

 no crystals as high in (A) as in the previous case (the first crystals would 

 have the composition n). If the solution reaches / or cools below it, 

 without the formation of any crystals, it will crystallize (if at all) in 



A 



Figure 6 



Composition during Melting in an Iso 

 morphous Series 



Concentration J} 



Diagram showing Change in 



