THE 



GEOLOGICAL MAGAZINE. 



NEW SERIES. DECADE V. VOL. VIII. 



No. VII. —JULY, 1911. 



OIRIG-IIEsr.AJL, ARTICLES. 



I. — The Fundamental Problems of Petkogenesis, oh the Origin 

 of thk Igneous Hocks. 



By Dr. Franz LffiWiNSON-LESSiNG, Professor of Mineralogy and Geology, 

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

 Soc. London. 



(Concluded from the June Number, p. 257.) 

 § 2. On the Causes of the Diversity of Igneous Rocks. 



THE second fundamental problem of pedogenesis is the following 

 question : — By what processes have there been derived, from the 

 original magma or magmas, all other magmas known as igneous 

 rocks ? It is now almost universally admitted that many igneous 

 rocks are genetically connected and produced from an original magma 

 by differentiation, and that in many cases differentiation may be 

 considered as liquation or separating into secondary magmas. But 

 is differentiation (Spaltimgen) alone sufficient to explain the formation 

 of all igneous rocks? And by what is a magma stimulated to 

 differentiation ? These are two questions which must be elucidated. 

 It must be first of all emphasized that not every magma is subject to 

 differentiation. My standpoint is that the principal factors producing 

 and regulating differentiation are on one side the process of 

 crystallization (differentiation by crystallization, Krystalli%ations- 

 differenzierung), and on the other the chemical composition of the 

 magma and its tendency to the formation of eutectics (magmatic 

 differentiation). 



Gravitation is the principal factor during differentiation by crystal- 

 lization : crystals are distributed according to their specific gravity, 

 the one rising to upper portions of the magma, the other sinking into 

 deeper and being often dissolved and assimilated there. Corrosion- 

 rims on phenocrysts and orbicular concretions in plutonic rocks, 

 well known to every petrographer, are suggestive illustrations of 

 the fact that there exist in the magma vertical movements of the 

 first products of crystallization. By this process leucocratic and 

 melanocratic modifications may be generated, and there can issue from 

 one original magma several different rocks genetically and chemically 

 connected with it. Such a process can take place in a separate 

 isolated magma-basin cooling very slowly. 



The tendency to form eutectic mixtures is certainly a powerful 

 factor in differentiation, which, as I have already pointed out, 1 is from 



1 F.Lce-winson-Lessing, "Petrographical Notices. 4. Differentiation, Eutectics, 

 and Entropy " : Ann. Inst. Polyt. St. Petersbourg, vi, p. 279, 1906. 



DECADE V. — VOL. VIII. — NO. VII. 19 



