Differentiation in Silicate Liquids. 

 The Effect of Convection Currents. 



183 



The cooling of a body of magma at its upper margin must 

 be considered to cause a greater density of the liquid near the 

 top and, therefore, to exert a tendency towards the establish- 

 ment of convection currents. We are acquainted with convec- 



Fig. 4. 



Fig. 4. Section from top of melt showing tridymite crystals in glass. 

 x 25. 



tion currents largely in their very vigorous form as set up in a 

 beaker of water when placed over a flame. Very little of a 

 quantitative nature is known relative to convection currents. 

 It appears, however, that a marked gradient of temperature, a 

 high coefficient of thermal expansion of the liquid and a low 

 viscosity should favor vigorous convection. When a beaker of 

 water is placed over a flame it is exposed to a very high tem- 



