DIFFERENTIATION OF IGNEOUS ROCKS 417 



Th. Vogt, in the lecture above quoted (April 16, 1920) in 

 Christiania Videnskapsselskab, mentioned that the contents of 

 light volatile compounds ordinarily must have been smaller in the 

 effusive magmas than in the corresponding deep-seated magmas. 

 Because the pressure increases the melting or the crystallization 

 temperature much less than the volatile compounds lower it, 

 therefore the deep-seated rocks in general, with perhaps some 

 exceptions, will have crystallized at some lower temperature than 

 the corresponding effusives. 



Even if we do not consider the contents in the magmas of the 

 light volatile compounds, the granitic magmas will show a lower 

 temperature for the beginning of the crystalUzation than the 

 magmas of any of the other more common rocks. Since we must 

 assume that there is a larger amount of H^O, CO2, etc., in the granitic 

 rocks than in the analogous magmas of the other more common 

 rocks, we may draw the conclusion that the granitic rocks are charac- 

 terized by lower temperature for the beginning of the crystallization 

 than are the other rocks, perhaps with the reservation that there 

 may be some rare igneous rocks of small extent where the temperature 

 of the solidification is just as low or perhaps even a little lower 

 than that of the granitic rocks. 



R. A. Daly' states, "that the ordinary granites have been at 

 least partly molten at a temperature no higher than 870°, nor 

 lower than 575°." The first mentioned figure (870°) applies to 

 the point of inversion between tridymite and a -quartz, but this 

 point increases very remarkably with the pressure.^ Consequently, 

 the point of inversion determined at i atmosphere pressure cannot 

 be used as a geological thermometer. The point of inversion 

 between a and /S quartz (at i atmosphere pressure = 575°) seems on 

 the other hand to increase only very Httle with the pressure.^ 

 In this way we get a lower boundary at the conclusion of the crystal- 

 lization. 



The interval of the crystallization of granite-magmas — ^presumed 

 waterfree and at i atmosphere pressure — ^is calculated to be between 



^Igneous Rocks and Their Origin, 1914, p. 214. 

 ' See this Journal, 1922, p. 620. 

 3 See this Journal, 1922, p. 619. 



