MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 243 



hours, days, or with extremely viscid masses, perhaps even for 

 weeks. 



This general fact has inter alia an important application to the 

 relation between the time of crystallization and the viscosity in smelting 

 masses of approximately eutectic composition. In the adjacent 

 diagram (Fig. 55), kl indicates a certain viscosity and mn at lower 

 temperature a higher degree of viscosity of the melt. When 

 undercooling is not taken into consideration, the mineral a in a 

 smelting mass of composition Wj will begin to crystallize at t^, in a 



Ta 



k 



m 



I 



a 



Fig. 55 



smelting mass of composition u^, on the other hand, at 4 — thus at 

 a stage when the molten mass has grown more viscous. It is the 

 same if in both cases we presume the same degree of undercooling. 

 If the cooHng proceeds at the same rate, we will get quite large 

 crystals and some glass from the Wi melt, while from the u^ melt 

 we will get glass with a small quantity of small crystals. At a still 

 quicker rate of cooHng, no crystallization at all takes place in u^, 

 so that it soHdifies entirely as glass. 



That crystalhzation actually takes place, as here theoretically 

 explained, I have proved experimentally in my previous works. 



