CA USES OF MA GMA Tl C DIFFERENTIA TION. yjj 



compounds forming the original rock magma are completely 

 soluble in one another ? I think not. 



We are told by Vogt 1 that silicates can be melted together 

 in all proportions. This may be true, but it does not prove that 

 this mixture would not separate into layers of different compo- 

 sition, or at least become heterogeneous, if it were kept molten 

 for a sufficient time. The viscosity of molten glasses is very 

 great and consequently the separation must take time. Still 

 evidences of such separation — or liquation as we may call it, 

 following Durocher — in the manufacture of glass are not wanting. 

 It is well known to be very difficult to produce large pieces of 

 homogeneous glass, for example for optical purposes. Accord- 

 ing to Wagner's Handbuch der chemischen Technologie 2 this 

 comes from the fact, " either that the individual compounds 

 formed during the melting process have not dissolved one 

 another or that they have separated from the mixture by a lower- 

 ing of the temperature " ; and further, " One will seldom find 

 large pieces of glass, which are completely free from this fault." 3 

 But it is not necessary to leave the field of geology in order 

 to decide the question whether magmatic differentiation is a 

 diffusion, or a liquation, process. Let us select some examples 

 of differentiation, and examine them in the light of both theories. 

 I have chosen two, one on a small scale, the basic inclusions, and 

 one on a large scale, the great petrographical province of 

 Iceland. 



By diffusion, according to " Soret's principle," the basic 

 inclusions could never be thought to have been formed in situ 

 or approximately so — for, between them and the surrounding 

 magma there would be no difference in temperature, or at least 

 no difference sufficient to alter the osmotic pressure, which is 

 proportional to the absolute temperature, or enough to produce 



1 Zeitschr. f. prakt. Geol., 1893, 2 7 2 - 



- 13th edition, 720. (Leipzig, 1889). 



3 " Entweder die einzelnen beim Schmelzprocesse entstandenen Verbindungen sich 

 gegenseitig nicht aufgelost, oder bei einem Nachlassen der Temperatur aus einem 

 Gemenge sich abgeschieden haben " ; and further, " Man wird selten grossere Stiicke 

 von Glas finden, welche von diesem Fehler vollkommen frei waren." 



