564 N. L. BOWEN 



diopside-bearing variety of the Beckett gneiss, of which Eskola has 

 written a description and interpretation/ I visited the localities 

 with Dr. Eskola, and in my opinion the transformation of the solid 

 dolomite into solid diopside with its effect upon the granitic liquid 

 was the dominant action in the production of the types of gneisses 

 there found, and of their banding, rather than actual solution of 

 the dolomite or skarn in the granite and subsequent differentiation of 

 the syntectic liquid. It must be admitted, however, that the solu- 

 bility of CaCOs in magmas is probably greater than that of CaO and 

 that under conditions permitting the retention of CO2 an amount of 

 limestone might be dissolved greater than that suggested by the 

 reaction effects already discussed. The usual free conversion of 

 limestone into silicates indicates, however, that it is not commonly 

 so dissolved. 



The formation of basic silicates, without the production of feld- 

 spathoids, seems to be the ordinary result of the action of granitic 

 magma on limestone inclusions. Thus, in the granitic portions of 

 the Bush veldt laccolith altered limestone inclusions are surrounded 

 by a halo of dioritic material, but not by alkaline rock.^ Other 

 examples might be given; in fact, the ordinary effects of granite on 

 limestone seem to be those we have deduced for a saturated granitic 

 magma. 



In one locality the alkaline facies of the Bush veldt complex is, 

 it is true, intimately associated with a mass of limestone, and as a 

 result of a study of this locality Shand has concluded that there is 

 some connection between the production of the feldspathoids and 

 the desilicating action of the magma. Apparently Shand does not 

 believe that the entire production of nephelite is due to this action 

 but rather that a nephelite syenite magma becomes ijolite by desili- 

 cation.3 This is a quite different matter from the production of the 

 original nephelite syenite magma by such desiUcation. No theoret- 

 ical objection can be raised against the belief that interaction with 

 limestone could reduce the amount of feldspar and increase the 



^ P. Eskola, Jour. Geo!., Vol. XXX (1922), pp. 265-94. 



* Oral communication. Professor Brouwer. 



3 S. J. Shand, Trans. Geol. Soc. South Africa, Vol. XXII (1921), pp. 144-46. 



