﻿272 



DR. A. HOLMES ON THE TERTTAKT 



[vol. lxxii, 



Muxes, even at moderately high temperatures, two suggestions may 

 be made : — . 



(i) that the portion of a magma specially rich in water and free silica — 

 and therefore light — tends to move upwards, crystallizing- only when 

 water begins to escape, and leaving a desilicated portion in which 

 the crystallization of nepheline would then naturally proceed ; or 



(ii) that in the presence of water, nepheline and alkali-felspar could 

 crystallize, leaving free silica in solution, the ratio between nepheline 

 and alkali-felspar depending on temperature and pressure and on the 

 concentration of water in the magma. 



For the reasons stated above, it is considered that the various 

 rock-types of Series A could be evolved by a process of crystalli- 

 zation accompanied by a gravitational settling of the heavier 

 crystals and an upward separation of the lighter residual magmas. 

 At intervals the process would be locally brought to an end by the 

 eruption of lavas. 1 



(b) Except at the close of the magma tic history described 

 above, the rocks produced were under-saturated with silica. The 

 parent magma must also have shared this characteristic, and other 

 features to be explained (in contrast with the magmas concerned 

 in the production of the amygdaloids) are the comparative poverty 

 in water and the presence of carbon dioxide. 



The well-known theory of Prof. P. A. Daly, 2 who ascribes 

 alkali rocks to the desilication of basaltic magma by absorption 

 of carbonate sediments, is quite inapplicable to the present case. 

 Certainly calcareous bands occur in the Cretaceous rocks of the 

 Sanhuti district, but these are merely a thin veneer on a Pre- 

 Cambrian basement, and can have had no effect on the composition 

 of the lavas. Crystalline limestones occur in narrow bands inter- 

 foliated with the gneisses of the basement rocks, but in such small 

 quantity that, if any part of them had been absorbed, a much 

 greater proportion of granite and gneiss would also have been 

 taken up, and the desired effect would thus have been nullified. In 

 one respect only is there a point of contact between Daly's theory 

 and the view here adopted : namely, the belief that carbon dioxide 

 was an important member of the volatile fluxes belonging to the 

 magma from which the alkali lavas were derived. 3 Under high 

 pressures caleite and magnesite can exist in the presence of silicate 

 minerals without dissociation if the temperature be not too high. 

 It is therefore possible that at very great depths, and among rocks 



1 The process here suggested is, in principle, not unlike that advocated by 

 Dr. N. L. Bowen (op. cit. 1915), except as regards the relationships of silica, 

 nepheline, alkali-felspars, and water. 



2 Bull. Geol. Soc. Am. vol. xxi (1910) p. 115 ; and ! Igneous Bocks & their 

 Origin ' 1914, chap. xx. 



:i Brof. C. N. Smyth has recently discussed the genesis of alkali rocks on 

 "the hypothesis that they are segregated from ' normal ' magmas by the 

 action of special pneumatolytic agents. Am. Journ. Sci. ser. 4. vol. xxxvi 

 (1913) p. 33. 



