672 J. H. L. VOGT 



course of the crystallization, at least in part, will have formed 

 miarolitic druses. These play a rather subordinate part in regard 

 to the quantity in the granite-pegmatite dikes, however. 



Even in the granite-magma the quantity of H2O will hardly have 

 amounted to much more than a few per cent. And the crystalliza- 

 tion of the igneous rocks must, as pointed out by many earlier 

 investigators, among them also myself, be conditioned by the 

 decreasing temperature of the magma and not by the diminution 

 of volume occasioned by the escaping H2O, etc. 



According to Clarke's well-known calculation the earth's crust 

 consists to a depth of 10 miles { = ca. 16 km.) at a medium density 

 of the rocks of 2.7, of 



93.39 per cent solid crust, with 2.02 per cent H2O 

 6.58 " " ocean, with ca. 97 per cent H2O 

 0.03 " " atmosphere 



If we were to presume that the entire quantity of water of the 

 ocean was supplied from the igneous magmas, and assume a medium 

 thickness from the solid crust of three times 10 miles = ca. 48 (or 

 50) km., we would get a quantity of H2O: 



1X6.58X0.97-1-97.81X2.02 = 4.06 per cent H2O. 



This value, ca. 4 per cent, must indicate the maximum original 

 content of H2O in the initial parent-magmas. But of this again a 

 considerable part must have escaped before the parent-magmas 

 could have been cooled so far that crystallization-differentiation 

 began. The various partial magmas, resulting from magmatic 

 differentiation, which, crystallized to form the igneous rocks, must 

 thus, on an average, have contained less, very likely quite consider- 

 ably less, than 4 per cent H2O. 



In concluding, I wish to note that I have learned much from P. 

 Niggli's great work : " Die leichtfliichtigen Bestandteile im Magma " 

 (1920), but I cannot endorse his statement (pp. 123-38) that the 

 volatile compounds have been the important factor in magmatic 

 differentiation. This will be more closely treated in a later paper. 



[To be continued] 



