MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 66i 



solidification of the magma in a depth, for instance, of 5 or 10 

 kilometers. And in the last case the escaping gas must dissolve 

 in itself more of the sili- 

 cate compounds. Th. 

 Vogt applies this to very- 

 instructive geological 

 examples. 



2. At the tempera- 

 ture interval Q-P there 

 exists a gas (H2O in su- 

 percritical condition 

 with some dissolved sili- 

 cate in gas-formed con- 

 dition) from which more 

 or less silicate may cry- 

 stallize directly from 

 the gas. 



3. From P (the criti- 

 cal point of H2O con- 

 taining some B in 

 solution, which point 

 lies somewhat higher 

 than the critical point, 

 Ka = 374'', of pure H^O) 

 to E (Fig. 52) or Sa (in 

 Fig. 53, where E and Sa 

 practically fall together) 

 we get a liquid very rich 

 in H2O, which may effect 

 divers hydrotermal for- 

 mations. Further on 

 we get a gas phase con- 

 sisting of quite predomi- 

 nant H2O and only a 

 very little B . 



We shall in the following pages only treat the crystallization 

 in a magma, containing light volatile compoimds, and not employ 



H^O B 



Fig. 53. — System A:B, where A=H20 and B = 

 silicate (silicates of the granite). Qu = quartz point 

 (inversion points between a and jS quartz, at one at- 

 mosphere at 575° and at pressure somewhat higher). 

 The pressure may be indicated by axes perpendic- 

 ular to the plane of the paper. At relatively low 

 pressure in the deep-seated magmas and conse- 

 quently at high temperature, contact gas escapes, 

 giving as a result the contact metamorphic ore de- 

 posits of the Kristiania type. At higher pressure 

 and consequently at a somewhat lower tempera- 

 ture, contact gases escape with a higher solution of 

 silicates, etc., giving, as a result, contact metamor- 

 phism of the Orijarvi type ("Orjv. gas"). After 

 Th. Vogt. 



By far-advanced crystallization of the granite 

 magma escape "cassiterite gases" (Sn F4, etc.), 

 giving as a result the minerals of the cassiterite 

 deposits. 



