DIFFERENTIATION OF IGNEOUS ROCKS 415 



eutectic boundary line between Qu and the feldspars, but since 

 the ordinary granitic rocks do not contain any significant surplus 

 of quartz, the rise will be rather small. 



We wUl now give a review of the temperatures (calculated at 

 I atmosphere pressure and without regard to the contents of H2O, 

 etc., in the magma) at the beginning of crystallization of the 

 silicate minerals for a number of igneous rocks. 



Anchi-monomineralic rocks. — ■ 



Dunite About isoo°>> occasionally 1550-1600° 



Other peridotites with less olivine. .x\bout 1400° 



Bronzite rocks About 1400° 



J Labradorite rocks About 1400-1450° 



\ Bytownite rocks About 1475-1500° 



Anchi-euteciic rocks. — 



Gabbro and norite About 1250° 



Diorite About 1 200° 



The most common syenites . . . .About 1100° 



The ordinary granites About 1000° (in part a little less) 



We will here have to make two corrections: The pressure will 

 occasion a rise, while the H2O, etc., content on the other hand, 

 will cause a lowering of the temperature of crystallization. The 

 effect of pressure in lava flows may practically be neglected, and 

 even in deep-seated rocks at 5 to 10 km. depth, the effect is very 

 insignificant (see my treatise in this Journal for 1922, pp. 611-14). 

 Even though the pressure of 5 to 10 km. depth raises the beginning 

 of the crystallization of one mineral say 10°, and another say 40°, 

 the difference between these figures is unimportant. 



Dift'erent, however, is the effect of the light volatile compounds, 

 H2O, CO2, etc., since these, when present in a noteworthy amount, 

 lower the temperature of the crystalUzation remarkably. This 

 lowering is nearly proportional to the amount of H2O, CO2 etc., in 

 the same magma. 



The content of H2O, CO,, etc., in the magmas of anorthosite, 

 dunite, bronzite-rock, and other petrographically related anchi- 



