248 Prof. F. Lmwinson- Leasing — Origin of the Igneous Rocks. 



(41) Eoth, Justus, Allgememe und Chemisclie Geologic, vol. iii, p. 140, 1890. 



(42) Teall, J. J. H., " On Greenstones associated with Eadiolarian Cherts " : 



Trans. Eoy. Geol. Soc. Cornwall, vol. xi, p. 560, 1895. 



(43) Ussher, W. A. E., " The Geology of the Country around Plymouth and 



Liskeard " : Mem. Geol. Surv., p. 83. 



(44) Fox, H., Jour. E. Inst. Cornwall, vol. xii, p. 34, 1895. 



(45) Johnstone, J., Conditions of Life in the Sea, 1908, p. 237. 



II. — The Fundamental Problems of Petrogenesis, or the Origin 

 of the Igneous Hocks. 



By Dr. Franz Lgewinson-Lessing, Professor of Mineralogy and Geology, 

 Polytechnic Institute, Sosnovka, St. Petersburg, Eussia; For. Corr. Geol. 

 Soc. Lond. 



Introduction. 

 rPHE question of the origin of igneous rocks, their diversity and 

 JL genetic relation ships, represents that fundamental problem of 

 petrography which has been for many years the object of inquiry for 

 penologists as "well as for geologists and chemists. Whilst the 

 amount and scope of detailed observation were growing, the methods 

 of experimental investigation improved, and as the eruptive rocks 

 came to be studied from the point of view of physical chemistry, so 

 the petrogenetical horizon became larger and wider. Thus, on the 

 basis of numerous minute and detailed observations were built broad 

 generalizations that gave rise to new problems. 



If we try to determine by one word the successively predominant 

 ideas in petrogenesis, we can say that, notwithstanding a certain 

 diversity of opinions, from 1890 and for nearly fifteen years petrology 

 was dominated by the idea of differentiation ; from the middle of the 

 last decennium the chief role belongs to the theory of eutectics ; and 

 during the last few years there has been slowly growing and advancing, 

 as a necessary corollary and even as a dominant factor, the hypothesis 

 of refusion and assimilation by fusion. A rational combination of the 

 principles of assimilation, refusion, differentiation, and eutectics seems 

 to be the best basis for a theory of the genesis and diversity of the 

 igneous rocks, and probably the nearest approximation to the solution 

 of the question may be my syntectic-liquational hypothesis l enlarged 

 according to the progress of later years. 



In the present paper 2 I propose to give my revised views on the 

 genesis of the igneous rocks and the factors governing their diversitj r . 



§ 1. The Average Chemical Composition of the Earth's Crust and 

 the Primordial Magma. 



Before entering into the problem of the genesis of the igneous rocks 

 it is necessary to deal critically with a question of general interest, 

 which has oftenbeen an object of inquiry for petrographers and chemists. 



1 F. Lcewinson-Lessing, " Etudes de petrographie generale," p. 178 : Trans. 

 Soc. Nat. St. Petersbourg, 1898. " Studien iiber die Eruptivgesteine," p. 187 : 

 Compt. rend. Congr. Geol. Inter., St. Petersbourg. 



2 Originally written in Eussian, August 27, 1910, and printed in the 

 Annales de l'Institut Polytechnique de St. Petersbourg, v, xiv, 1910, p. 111. 



