4i8 /. E. L. VOGT 



about 1000° and 900°. In deep-seated magmas we will get a small 

 addition to this, say 10-50° on account of the pressure, but on the 

 other hand, a greater subtraction on account of the hght volatile- 

 compounds. Therefore the crystallization will probably not have 

 started at a higher temperature than about 950°, and for the 

 relatively HjO-rich anchi-eutectic magmas (delivering biotite and 

 muscovite) we will probably have to count- on only 900°. The 

 conclusion of the crystallization may have taken place at a little 

 lower temperature. The sohdification of the granite therefore lies 

 at about 900°; probably, for many granites even a little lower, say 

 between 900° and 800°, 



As to the granite-pegmatite dikes, with relatively large quantities 

 of the light volatile compounds in the magma we will have to count 

 on still lower temperatures; estimated by Th. Vogt at about 700°. 



The great difference in the temperatures of the solidification 

 of different igneous rocks can be illustrated as foUows: two-thirds 

 or three-fourths of an anorthosite magma solidifies at a temperature 

 at which a gabbro-magma is still quite fluid, and a quartz-free 

 gabbro is completely solidified at a temperature at which a granite 

 magma has not yet started to crystallize (perhaps not considering the 

 beginning of crystallization of certain telechemic minerals such as 

 apatite, zircon, pyrite, etc.). 



We will in conclusion point out the following: i. The granites 

 have the lowest melting or crystallization temperatures of all igneous 

 rocks (at least of all the more common rocks). 2. As an end- 

 product by the sohdification of gwarfe-bearing gabbros, norites, 

 syenites, etc., there results a granitic magma, which has often 

 burst forth in the form of special dikes. 3. Granites belong to 

 the last epoch of the eruption in the great petrographic provinces. 



Points 2 and 3, as will be explained in a later paper, are due to 

 the fact that magmas containing Qu as a special component (corre- 

 sponding to more than about 55-60 per cent Si02 in the total rock) 

 give by a differentiation of crystallization, according to our physico- 

 chemical melting-diagrams, a granitic magma as the end-product. 



On account of the physico-chemical facts we may draw the 

 conclusion that this end-magma is characterized by a relatively 



