[ADAMS 4 barlow] ALKALI SYENITES OF EASTERN ONTARIO 65 



CO2 in the rock, the results given under II are obtained. This is the 

 composition of the residual or alumina saturated magma. 



Morozewicz' has shown by direct experiment that in supersaturated 

 alumino-silicate magmas, whose general composition is EO, m AI2O3, 

 n SiOj (where R = K2, Naj or Ca, and n ^ 2), the whole of the excess 

 of alumina separates out — (1) as corundum, if no considerable amount 

 of MgO or FeO is present; and if n is less than 6. (3) as sillimanite 

 and corundum; if n is greater than 6. (3) when the magma is rich in 

 magnesia^, as spinel or spinel and corundum; if n is less than 6. (4) as 

 eordierite, or cordierite with one or more of the other minerals, if n is 

 greater than 6. The absence of corundum in the neplieline syenites of 

 India is explained by Holland as due to the fact that these rocks, as 

 shown by analysis, contain too much MgO and FeO; and he refers to the 

 abundance of iron magnesia minerals in the nepheline syenite and the 

 scarcity of such minerals in the corundum syenite as amply accounting 

 for the abundance of free alumina in tlie latter and its absence in tlie 

 former. A similar low content of iron and magnesia is noticeable in the 

 Canadian corundum syenites, and, together with the high percentage of 

 alumina in the magma, probably explains the development of corundum 

 in them. 



The ratio of the molecular values of (CaO, K^O, NaaO) : AUOg : 

 SiOo in the rock at present under consideration are as follows: — 



.228 .233 .972 



1 : 1 : .42 



The ratio of K^O : Nag 0=1. 6 and that of alumina to the bases is a 

 little in excess of 1:1. As a magma for the solution of alumina and its 

 complete separation as corundum on crystallization, it is therefore in per- 

 fect agreement with Morozewicz's law. Of the alkalis, soda largely pre- 

 dominates, this lending the necessary assistance in the solution of the 

 alumina. There is an excess of ferrous iron and magnesia, above what 

 has been thought permissible (0.05 per cent) by Morozewicz's law, but 

 these amounts have been necessary to assist in the formation of the 

 comparatively small quantities of magnetite and biotite present in the 

 rock. It is therefore evident that Morozewicz's law, as remarked by 

 Holland,^ does not represent the whole truth, for it might be expected, 

 with this excess of ferrous iron and magnesia, that spinel would be 



^ Tscher. Mitt. Band XVIII (1898)— pp. 1-90 and 105-240. 

 'Mem. Geol. Surv. Ind. Vol. XXX, part 3. (1901) p. 208. 



Sec. IV., 1908. 5. 



