WARREN. — ALKALI-GRANITES AND PORPHYRIES. 317 



not precede differentiation.^^ Crosby's final statement regarding 

 their origin is as follows ^'': — ". . . . segregation on a large scale and 

 subsequent crystallization developed a continuous zone of basic- 

 porph;\ry: and that as the energy of the process diminished, or 

 where it was primarily weak, it became more or less localized, 

 developing isolated segregations which were subject to various acci- 

 dents — distortion, cracking and crowding — during the gradual stiff- 

 ening of the enclosing magma." The writer is disposed to insist 

 that the evidence, as above given, shows that the segregation referred 

 to by Crosby was confined to the immediate contacts and that it did 

 not form small localized masses (the size of the present xenoliths) in 

 the magma at points distant from the contact. 



The Relations between the Soda and Potash Feldspars. — In the 

 descriptive part of this paper it has been noted that the feldspar which 

 occurs in the form of phenocrysts in the porphyries, when unaltered, 

 is a soda-orthoclase (?) — or a very fine cryptoperthite. The pheno- 

 crysts consist centrally of a homogeneous material or a very fine 

 (almost homogeneous) cryptoperthite, but toward the margins they 

 become more distinctly perthitic though the structure is still very fine. 

 In the groundmass of the rhombenporphyry and in the later rims of 

 groundmass age about the phenocrysts, we have cryptoperthite or 

 coarser microperthite ; in the granite-porphyry the rims about the 

 phenocrysts are of microperthite as are also the small crystals in the 

 groundmass. In the granites, on the other hand, the feldspar is 

 throughout a strongly developed and fairly coarse microcline-micro- 

 perthite, with a very subordinate amount of separately crystallized 

 albite, and this is located about the ends or sides of the microperthite 

 crystals, usually as a continuation of the albite within the body of the 

 crystal. 



Recently J. H. L. Vogt ^^ in an elaborate paper, has developed the 

 view that the anorthoclase — cryptoperthite — feldspars are eutectic 

 mixtures and proposes to designate them as "eutectic feldspars." 

 As the result of a statistical study of the available chemical analyses of 

 feldspars and the rocks in which they occur, he has computed that the 

 eutectic mixture lies between the limits 40-44 Or: 60-56 Ab -\- An; 

 that the mixed crystal phases forming the eutectic are about Or, 

 12% : Ab + An 88%, and 72%^ Or : 28% Ab + An. That is, he believes 



76 op. cit., p. 372. 



77 op. fit., p. 373. 



78 Tschermak's Mineral, u. Petrog. Mitt., 24, No. 6 (1906). 



