Tin: OKICIN OF MINI I;AI.S 243 



heated to a temperature ;ilx)ve 32 the anhydrous salt N:uSO 4 

 will separate at 33. The solution is carefully poured off the crys- 

 tals in a flask ;uid the mouth of the Husk stoppered with cotton to 

 prevent the accessof dust or fine p.-irt ides of sodium sulphate from 

 the air. which would start crystallization and prevent supercooling 

 or supersaturation. Such a solution may be cooled to room 

 temperature and kept for a long time without the formation of 

 crystals. This solution is supersaturated in regard to the salt 

 Na_.SO 4 , 10 H 2 O ; if the smallest particle of this salt is dropped into 

 it, the solution solidifies almost at once, with a rise in temperature, 

 and the system soon reaches a state of equilibrium, as between the 

 solid NaaSC^, 10 H 2 O and solution. 



The supercooled solution is in the metastable state, and the tend- 

 ency to form crystals spontaneously is very small; if cooling is 

 continued, however, a temperature will be reached at which a cloud 

 of small crystals will form spontaneously, or a number of crystal- 

 line nuclei will form without inoculation with a solid particle, and 

 thereafter the system quickly reaches equilibrium. 



In the case of igneous rocks, before consolidation they may be 

 said to represent solutions of various components, each of which will 

 have an individual and separate influence on the temperature of sep- 

 aration of every other dissolved component in the magma, and the 

 freezing curve, or the curve of separation of any crystalline form 

 or phase separating, will be the resultant of all these depressions. 

 Where the number of components is quite large, six or eight, as 

 it is in most magmas, the system becomes excessively complicated. 

 Upon the whole, when cooled to the metastable stage of one or more of 

 the components, a temperature will be reached at which crystalline 

 nuclei will form spontaneously, and the minerals will separate from 

 the magma in the order of their freezing points or saturation 'for 

 that particular system, and each individual crystal will continue to 

 jjrow as cooling of the magma continues. In the fusion diagram, 

 fields III and IV, other things being equal, would represent a 

 porphyritic development of one or two species; the individual 

 crystals of each would be contained in a ground mass of fine crys- 

 tals, representing the final disappearance of solution, or the eutectic 

 mixture composed of the last to crystallize, Fig. 369. The eutectic 

 mixture in its crystallization always presents a peculiar, intimate 

 mtergrowth of the components. Thisisseeninthemicropegmatitic 

 Bitergrowths of quartz and orthoclase, Fig. 370. Eutectics are 

 often formed Between garnet and quartz ; magnetite and horn- 



