156 Scientific Proceedings, Royal Dublin Society. 



ausgehen." He points out in a later passage the analogy between concretions 

 in sedimentary rocks and orbicular spherulites. The features shown by the 

 spherulites of Mullaghderg certainly demand a movement in opposite 

 directions, which continued to act even after the formation of well-marked 

 zonal envelopes. 



The grey zone of radially arranged soda-lime felspar round about the granitic 

 and schistose nuclei at Mullaghderg corresponds with the zone of mixture 

 described by Frosterus and other authors. 47 It lies at some distance from the 

 relics of the original nucleus, on account of the inward flow of the granite 

 magma and the outward flow of basic matter through it. At a certain 

 distance from the original nucleus of amphibolite or biotite- schist, the 

 ferromagnesian compounds have decomposed, and iron has separated in the 

 form of magnetite, which was immiscible with its molten surroundings at the 

 temperature attained. 48 From the close association of biotite with the 

 magnetite, and especially its occurrence in one of Miss Andrews's specimens 

 as a zonal deposit along with the magnetite grains, it appears that biotite 

 separated from the mixture, and was then decomposed and in large part re- 

 absorbed. The withdrawal of iron oxide from the molten mixture, 

 impoverishing the zone immediately outside that along which the magnetite 

 was deposited, appears to have allowed of further diffusion, without more than 

 a scattered crystallisation of magnetite. But soon a second deposition 

 occurred, and the process was repeated rhythmically, until in some cases four 

 light-coloured zones and four more rich in magnetite were formed. 



Ultimately the mixed silicates in which the magnetite granules made 

 their appearance crystallised radially as andesine, or oligoclase verging upon 

 andesine. The radial structure of the felspar probably accounts for the 

 arrangement of much of the magnetite, which developed as a number of 

 accreting nuclei between the main concentric zones of deposition. A small 

 amount of biotite occurs in the felspathic envelopes, showing that some of the 

 material of the nucleus escaped complete decomposition. The great bulk of 

 the magnesium, however, and perhaps part of the lime, passed out from the 

 destroyed portion of the inclusion into the general magma of the cauldron. 



In many cases, as we have seen, the process of zone-building ceased, on 

 account of the complete destruction of the basic inclusion. In other cases, a 

 relic still remained when the urgent attack of a fresh unsaturated magma, 



47 On the influence of temperature in freeing this zone from ferromagnesian consti- 

 tuents, compare J. A. Bancroft, op. clt. (32), p. 101. 



4S For numerous examples of the production of magnetite by the interaction of molten 

 minerals, see F. W. Clarke, "Data of Geochemistry," U. S, Geol. Surv., Bull. 616 

 (1916), p. 315. 



