GEOPHYSICAL LABORATORY. 1 



By Arthur L. Day, Director. 

 THE DIFFERENTIATION OF THE MAGMA. 



The crust of the earth, so far as it is known, is made up almost exclusively 

 of rocks formed by the consolidation or crystallization of molten matter — 

 the so-called igneous rocks. There is, in addition to the igneous rocks, only 

 the relatively insignificant veneer of detrital material resulting primarily from 

 their disintegration under the action of the weather. These fundamental 

 igneous rocks have naturally occupied the attention of the geologist since the 

 beginnings of his science. 



One of the earliest features recognized in connection with them was the 

 fact that the igneous rocks are not all alike, though associated so intimately as 

 to indicate a common origin. They vary from mass to mass and from place 

 to place within a single mass. Apart from the purely scientific interest in 

 this diversity of types, it is of importance in other respects. It is found, for 

 example, that some types give rise to a soil capable of supporting a rich 

 vegetation and others to a soil that is nearly barren. Certain metallic ores 

 are found in association with some types, while other types contain quite 

 different ores or none at all, so that the processes that have led to the differ- 

 entiation of the various types of igneous rocks really lie at the root of most 

 human activities. 



It is becoming increasingly apparent that the production of a number of 

 types of igneous rocks from a single molten mass must be connected with the 

 process of crystallization. If a certain substance crystallizes out from a 

 liquid mixture containing it, the remaining liquid must perforce be impover- 

 ished in that substance. The mere fact that the substances contained in 

 any such liquid mixture must crystallize from it in a certain order is, therefore, 

 sufficient to indicate variation of the crystalline mass from place to place. 



The large number of investigations of mineral systems that have been 

 carried out at this Laboratory furnish complete information concerning the 

 crystallization of liquids belonging to certain of these systems and the kind 

 of variation to which crystallization may give rise. Synthetic studies of these 

 systems have revealed the existence of a principle of first importance in 

 explanation of the variation of composition in igneous-rock masses. It has 

 been noted in a number of these systems that after the separation of crystals 

 of a certain kind the remaining liquid may later, in the cooling process, react 

 with these crystals to modify or completely change their composition. Such 

 reactions must likewise affect the composition of the liquid, so that the further 

 course of the liquid, the crystalline products that form from it, and the 

 temperatures at which they form will be affected in proportion to the freedom 

 with which these reactions take place. The conditions attendant upon 

 crystallization in turn govern the freedom of these reactions, so that through 

 the working of the reaction principle the crystalline products of a magma 

 are rendered responsive to surrounding conditions, particularly to the rate 

 of cooling. 



1 Situated in Washington, District of Columbia. 



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