rEBEUART 5, 1909] 



SCIENCE 



213 



more than two kinds of separating com- 

 pounds, may take place in solutions of 

 whatever composition. Eutectic mixtures 

 may consist of more than two components. 

 Moreover, the supersaturation of a solu- 

 tion by one component may affect the 

 proportion between two or more com- 

 ponents at the moment of synchronous 

 crystallization. Synchronously crystallized 

 mixtures of certain kinds of components, 

 therefore, are not necessarily similarly 

 proportioned. The bearing of these prin- 

 ciples on the crystallization and texture of 

 igneous rocks is manifold. A few illus- 

 trations will suffice. Quartz may be the 

 first mineral to separate from a molten 

 magma when the solution is so rich in 

 silica that upon cooling it becomes satu- 

 rated with silica before being saturated 

 with feldspar or some ferromagnesian 

 compound, or even iron oxide. Quartz 

 may be the last mineral to separate from 

 magmas so rich in feldspar or ferromag- 

 nesian compounds as to become saturated 

 by these upon cooling before being satu- 

 rated with quartz. 



Either labradorite or augite may sepa- 

 rate first from a mixture of the two, ac- 

 cording to which saturates the solution first 

 upon cooling, and this depends on their 

 relative amounts in the solution, and their 

 order of crystallization is further modified 

 by the possibility of one or the other pro- 

 ducing supersaturation in the liquid. 

 This will account for the differences of 

 texture often noted in certain gabbros, or 

 basalts, of almost the same composition. 



Eutectic mixtures, or those whose com- 

 ponents crystallize simultaneously, often 

 yield aggregates of intergrown crystals, the 

 most familiar examples of which are found 

 in graphic granite, and certain alloys. But 

 Miers has called attention to the fact that 

 the simultaneous crystallization of two 

 compounds in eutectic proportions does not 

 invariably produce intergrown individual 



crystals, or graphic intergrowths. It may 

 result in granular aggregations of con- 

 certed, adjacent anhedrons not intergrown, 

 which corresponds to observations on the 

 textures of igneous rocks, for many rocks 

 of like composition in some instances ex- 

 hibit graphic texture, in others evenly 

 granular texture. Accepting graphic in- 

 tergrowth as evidence of synchronous 

 crystallization, and of the existence of 

 eutectic proportions in some cases between 

 the several mineral compounds at the 

 moment of crystallization, it is to be noted 

 that such intergrowths have been developed 

 in igneous rocks between quartz and 

 potash-feldspar, quartz and sodic feld- 

 spars, quartz and biotite, feldspar and 

 pyroxene, feldspar and hornblende, feld- 

 spar and nephelite, pyroxene and iron 

 oxide (probably magnetite) and between 

 other pairs of minerals. 



The separation of solids, that is, the crys- 

 tallization of minerals from rock magmas, 

 must be an extremely intricate process, 

 because of the complex character of the 

 solution, the variable and irregular changes 

 in temperature and pressure consequent on 

 the movements of eruption, the variations 

 in composition due to changes in gaseous 

 components, and the possibility of chem- 

 ical reaction among the components with 

 changes of chemical equilibria, as well as 

 the probable supersaturation of the magma 

 by different components to various degrees. 

 This complexity will doubtless prevent 

 exact statements of the relations between 

 composition and texture, but approxima- 

 tions may be made to the proper explana- 

 tion of some of the most common and char- 

 acteristic textures, which wiU render them 

 more intelligible to the student. 



The crystallization of a substance from 

 solution involves molecular diffusion, and 

 molecular orientation, and these are func- 

 tions of molecular attraction, composition 

 of the molecular compound, viscosity of the 



