8 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS. 



there can be only a limited number of isomorphous series and a limited 

 number of eutectic combinations. The most important isomorphous 

 series forms the subject of this publication. It is well known to be 

 probable also that the amphiboles, the pyroxenes, and the micas each 

 constitute isomorphous groups. No other isomorphous groups ap- 

 pear to have much lithological importance. In general, it is manifest 

 that isomorphism is to be expected only within groups of closely 

 allied compounds, and it is even a matter of surprise that the ortho- 

 silicate anorthite and the polysilicate albite should exhibit complete 

 isomorphism of the simplest type, as Messrs. Day and Allen have 

 shown that they do. It seems hardly possible, therefore, that a satis- 

 factory classification of rocks can be based on the study of isomor- 

 phous series ; indeed the mineralogical rock definitions of twenty-five 

 years ago were little else than such a classification, which has been 

 rejected as inadequate. 



No serious attempt has yet been made to group rocks on eutectic 

 principles, one very sufficient reason being our ignorance of eutexia in 

 magmas. Professor Lagorio refers briefly to eutexia, but regards the 

 important solvent in magmas as a silicate of the alkalies, the glass 

 least subject to devitrification.* Mr. Teall, in 1888, discussed eutexia 

 very lucidly and showed its importance in the physics of rocks, f but 

 he did not propose employing it as a basis of classification. In 1901 

 I briefly set forth some of the advantages of such a system. % 



The applicability of eutexia to rock classification depends upon the 

 fact that it makes the systematic discussion of magmatic mixtures 

 possible. Inasmuch as the subject-matter of lithology consists of 

 mixtures, their classification must be carried out in terms of definite 

 or standard mixtures, while the only mixtures possessing appropriate 

 distinguishing properties are the eutectics. Thus in dealing with 

 magmas or other heteromorphous miscible liquids the eutectics seem 

 to afford not only the best but the only natural and rational standards 

 of reference. With any eutectic as a basis, a series of magmas may 

 be prepared, each differing from the eutectic by containing an excess 

 of one or more constituents. Thus if abc represents an eutectic of 

 three substances, a mixture composed of a, mb, and nc may be re- 



* There is no fundamental difference between fluid solvents and solutes, and no 

 objection to regarding the alkaline glass as the solvent if found on other grounds 

 expedient. 



f Brit. Petrography, 1888, p. 394. 



% Report on the geology of the Philippine Islands. Twenty-first Ann. Rep. 

 U. S. Geol. Surv., Pt. Ill, 1901, p. 519. 



