from the Standpoint of Physical Chemistry. 219 



there shall be little or no reaction going on within the magmatic 

 solution from the time that the initial crystals appear until the 

 process of solidification is complete. In such cases the pro- 

 gress of solidification is very regular. Each compound present 

 in the fusion has its temperature of solidification depressed 

 according to the number of mols (gram-molecules) of other sub- 

 stances present and begins to appear at the appropriate point in 

 the process of cooling. That mineral will first appear which, 

 under the conditions stated regarding fusing-point, still has the 

 highest temperature of fusion. 



The first mineral may be regarded as that which is present 

 in greatest excess over the eutectic ratio. Within a certain 

 range of temperature it alone will be thrown out of solution. 

 At a certain point, however, it will be joined by a second min- 

 eral, and these two in turn by a third. The composition of the 

 solution approaches the eutectic ratio by the elimination of those 

 constituents which are in excess. A sudden chill at any stage 

 of the process causes a great increase of viscosity, which acts as 

 a very effectual check to further crystallization. The result is 

 the production of the greatly undercooleci liquid of immense vis- 

 cosity which is termed a glass. The composition of the glass 

 depends upon the stage of progress toward the attainment of 

 the eutectic ratio which has been reached. If no such inter- 

 ruption occurs, at the eutectic point the group of minerals form- 

 ing the eutectic will crystallize out in the proper ratio. ^No 

 further depression of the temperature of solidification can 

 occur, and the loss of heat will be merely that due to the latent 

 heat of fusion given up by each mineral in passing from the 

 liquid to the crystalline phase. An application of the phase- 

 rule of Gribbs confirms this conclusion, for at the eutectic point 

 the number of phases exceeds the number of components by 

 one, and no change of temperature or composition of the 

 system can occur. 



It is doubtful whether the solidification of a magma is ever 

 quite such a simple process as is expressed in the form de- 

 scribed, but though complications may ensue, the underlying 

 principles of eutectics hold and should constitute a guide of 

 great value in interpreting rock structures and history. In 

 the solidification of the Watchung magma the reactions which 

 would tend to obscure the process were of such nature that 

 their results do not offer great obstacles. The only one of 

 moment is that by which olivine was crystallized out and later 

 was resorbed by the magma and did not again appear. This 

 phenomenon will require explanation, but it is due to other 

 physico-chemical laws, and the reaction was so nearly complete 

 before the three final constituents, diopside, plagioclase, and 

 magnetite, began to appear, that its effects may be disregarded 

 for the present. 



