SECTIONAL TRANSACTIONS.— C. 417 



up to 13 %. The gneisses and other rocks around the vents are intensely 

 altered, the end-product being a feldspar-pyroxene rock in which the 

 pyroxenes are asgirine and aegirine-augite. These rocks correspond to the 

 fenites and tveitasites of the Fen district in Norway. The fenitisation 

 seems here to be due to emanations accompanying the emplacement of the 

 limestones and feldspathic breccias rather than to the small intrusions of 

 alkali-rocks (foyaite, ijolite, and nephelinite) which cut the vents and are 

 clearly later. 



It does not seem possible to explain the crystalline limestones in these 

 vents as due to carbonate replacement. They seem to be of magmatic 

 origin and to be comparable to the magmatic limestones or carbonatites of 

 Alno in Sweden and the Fen district. Their origin and their mode of 

 emplacement are problems which still await solution, and these problems 

 may be related to the problem of the origin of the associated alkali-rocks. 



Mr. S. I. ToMKEiEFF. — The rSle of carbon dioxide in igneous magma. 



All igneous rocks are known to contain CO 2 in varying quantities, but it 

 is probable that the greater part of the COj originally present in any magma 

 escaped during the consolidation. Certain rocks and rock-series are especi- 

 ally rich in CO 2, and given suitable conditions during the last magmatic 

 stages, not only did the carbonates crystallise out, but they formed an 

 independent carbonatite magma-fraction. Many occurrences of such 

 igneous carbonatite rocks are known, the one best studied being the Fen 

 district of Norway. 



The circular outcrop of alkaline and carbonatite rocks of Fen probably 

 represents the stoped head of a pipe infilled with a basic alkaline differentiate 

 of essexitic magma (Oslo district type). One may postulate that the upper 

 zone in the pipe was originally composed of an alkali-pyroxenite magma 

 rich in CO2. Crystallisation-differentiation, combined with diffusion of 

 alkalies and volatiles, gave rise to three main rock-series : 



(i) Urtite — Jacupirangite series. 



(2) Iron-ore series (Rodberg — Hematite ore). 



(3) Ca-Carbonate series (Kasenite — Sovite). 



The magma of the principal series (Urtite — Jacupirangite) assimilating 

 the country rock (Granite) gave rise to a hybrid alkali-syenite magma 

 (Juvite-Tveitasite series). The bordering gneiss-granite transfused by 

 alkali-alumina emanations derived from the main magma was transformed 

 into a pulaskite rock (Fenite). The residual liquid of differentiating magma, 

 rich in CO^ and iron oxides, gave rise to the late consolidated fractions of 

 iron ore and carbonatites. The evidence both of its field occurrence and 

 of its petrographical characters of Damkjernite suggests that this rock had 

 been derived from a lower zone of the pipe composed of alkali-peridotite 

 rich in COg. The shattering of the consolidated portion of this magma by 

 the residual volatiles and its subsequent eruption through the rocks 

 belonging to the earlier stage, gave rise to the Damkjernite — Rauhaugite 

 series rich in Mg and K. 



The r61e of CO2 in magma is not limited to the formation of magmatic 

 calcite and carbonatites. It is quite obvious that the presence of CO, 

 affects the equilibrium between other components in the magma and in 

 this way determines the formation of minerals. One may suppose that the 

 spilite-keratophyre series, and probably the lamprophyre series as well, 

 owe their peculiar character to the presence in them of a relatively large 

 amount of COj. The spilite-keratophyre series in its chemical composition 



