Volcano (JsLim.i, Llzii. 125 



liquid in contact with them, would change their compositions toward 

 enrichment in the lower-melting components. But slow cooling 

 affords a great opportunity for the continual movement of the 

 growing crystals with respect to the residual liquid unless they 

 match the liquid closely in density. Tlie augite crystals being 

 much heavier than the magma, their subsidence in it is beyond 

 dispute. The plagioclase crystals are nearly equal in density with 

 the magma, so that they must remain suspended in the liquid and 

 accomplish a change in composition, becoming more and more sodic. 

 In the meantime, the separation of these crystals would bring about 

 in the residual liquid a relative concentration of silica (pp. 108 S: 

 120-121) and the liquid would become gradually lighter as crystalli- 

 zation proceeded and at last decisively lighter than the plagioclase 

 crystals. Then the plagioclase crystals hitherto suspended would 

 begin to sink.'^ 



If the* cooling of the magma is rather more rapid, the sinking 

 of the crystals would be somewhat restricted. In this case the 

 plagioclase which does not differ n:iuch from the surrounding liquid 

 in density would have only a limited opportunity for sinking, and 

 the zoning of plagioclase would result. The augite crystals being 

 heavier than the plagioclase crystals, the former would subside from 

 the liquid in the upper portion before the newly separated crystals 

 could deposite around them, so that the augite crystals in the 

 upper portion would be only those rich in lower-melting component. 

 Then the resultant mass would consist in the upper parts mainly 

 of zonal structured plagioclase, lower-melting augite, and magnetite, 

 with the latest crystallization product as the interstitial matter. 



In our micro-diorite ; the plagioclase is from labradorite to 



1) Such a process was supjjosed also by Bowex when he discussed the anorthositcs (Jow: 

 Geol. Vol. XXV., p. 213, 1017). 



