THE FOYAITE-IJOLITE SERIES OF MAGNET COVE: 

 A CHEMICAL STUDY IN DIFFERENTIATION. 



INTRODUCTORY. 



Sometime since I published a paper ^ on the "Igneous Com- 

 plex of Magnet Cove," in which it was shown that the main types 

 found there were arranged in a very regular series from the center 

 to the periphery of the mass, and that this was an excellent 

 example of the differentiation of a magma in place, presenting, 

 however, the anomaly of being less "basic" at the borders than 

 at the center. It was also remarked that the analyses then 

 available "vary continuously in one direction, with scarcely a 

 break or abnormality of any kind."^ 



Since then several considerations led to the belief that a new 

 and more detailed chemical examination of the main rock types 

 was desirable. Several of these, notably the " leucite-porphyry " 

 and ijolite, are representatives of rock groups of great theoretical 

 importance, complete analyses of which are highly desirable. In 

 this respect many of those published by Williams^ are defective, 

 the non-determinations of the rarer constituents being largely 

 due to the fact that the importance of completeness in rock 

 analysis was not recognized at the time they were made.^ 



Such a reexamination seemed to be the more desirable, since 

 in a recent paper Pirsson^ has shown that the rocks occurring in 

 the Little Belt Mountains of Montana form an extremely regular 

 series. By plotting the constituent oxides on an abscissal basis 

 of distance from the center of the mass, he arrived at the con- 

 clusion in this case that, "given the percentage of one element, 

 the chemical composition of any rock of the series to within a 



^ Bull. Geol. Soc. Amer., Vol. XI, p. 389, 1900. ^ Op. cit., p. 403. 



3J. F. Williams, "Igneous Rocks of Arkansas." Ann. Rep. Geol. Surv. Ark. 

 Vol. II, 1890. 



^ Cf. J. C. Branner, in Williams, op. cit., p. xiv. 



5 L. V. PiRSSON, Twentieth Ann. Rep. U. S. Geol. Surv., Part II, p. 569 ff., 1900. 



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