112 H. S. JEVONS, H. I. JENSEN AND C. A. SUSSMILCH. 



In this part of our communication we propose therefore, 

 to seek the explanation of the origin of the various rock 

 types we have described, assuming that all have originated 

 in situ during the cooling and consolidation of a single 

 magma, which may, or may not, have been homogeneous 

 when intruded. 



This paper is divided into two parts as follows : — 



Part I. Differentiation Hypotheses by H. Stanley Jevons. 



Part II. The Explanation of the Differentiation Phe- 

 nomena of the Prospect Intrusion by H. I. Jensen 

 and C. A. Sussmilch. 



Part I was completed some live years ago, but its publi- 

 cation, for various reasons which it is not necessary to 

 enter into, has been delayed until now. 



Part I. Differentiation Hypotheses. 



By H. Stanley Jevons. 



Since there is as yet no consensus of opinion as to the 

 origin of igneous species, it is necessary for me in this 

 section to take a brief general survey of the various pro- 

 cesses of differentiation which have been advanced in a 

 general or particular manner, and to indicate the relative 

 degrees of importance which I attach to them. My resume 

 of the subject is to a large extent based upon the presi- 

 dential address on " The Evolution of Petrological Ideas," 

 delivered before the Geological Society of London in 1901, 

 by the present eminent Director-General of the Geological 

 Survey of Great Britain. 1 



The following is a statement of the principal modes of 

 origin of igneous rocks which have been, or may be sug- 

 gested with a considerable probability of truth. 



1 J. J. H. Teall, Section on " The Origin of Species," Q.J.G.S.. 1901, 

 pp, lxxviii -lxxxvi. 



