152 Scientific Proceedings, Royal Dublin Society. 



granite that here and there penetrates and parts the concentric shells sends 

 off feeders through these shells into the central miniature cauldrons. 



III. The Zonal Structure of the Spherulites. 



Zones, lighter or darker than the central core, have often been observed 

 round bodies included in igneous rocks. The zoning of ordinary crystals 

 through changes in the condition of the magma round about them is familiar 

 enough, and von Chrustschoff 3 ' observes that in the rock of Fonni felspars 

 which have been melted out as relics from porphyritic inclusions become zoned 

 by additions as they float away into their new surroundings. Von Chrust- 

 schoff also cites zones of hornblende round nuclei in a rock in the Svartdal 

 as due to the attraction of concentrated ore-masses in these nuclei. But the 

 outer zone in the case of most inclusions is not to be ascribed to deposition 

 from without, but to interaction between the inclusion and the enclosing 

 magma. Hence it frequently represents a loss, rather than an addition, of 

 material. 



Benedicks and Tenow emphasised the relict character of such zones in a 

 paper somewhat earlier than that already quoted. 38 In their considerations 

 the enveloping magma is supposed, for the sake of simplicity, to be of the 

 same composition as the inclusions, while the inclusions consist of two 

 minerals, say hornblende and felspar. It is suggested that such an inclusion, 

 as the temperature rises, loses by melting at its outer zone material which 

 will form, at that particular temperature, a eutectic layer. Material more 

 acid or basic than the eutectic remains behind nearer to the nucleus of foreign 

 rock. If the temperature continues to rise, the eutectic zone approaches the 

 original composition of the nucleus, by absorption of the relict zone, and at 

 last the inclusion may disappear altogether into the general melt. Cooling 

 at a particular stage, however, may preserve a distinctly zonal structure. 

 The authors urge that there is every passage in the granite of Upsala from 

 dark zones formed round fragments of mica-gneiss or diorite into the lumps 

 that Hogbom styles basic segregations. Two years later 33 they mixed paraffin 

 with graphite, and plunged angular blocks of this mixture, heated to near 

 their melting-point, into already molten paraffin. The paraffin streamed out 

 of the surface-layers of the block, leaving behind a dark zone of concentrated 



31 Op. cit. (5), pp. 233 and 239. 



32 "Om de s. k. basiska utsondringarna i Upsalagraniten och om klotgranitens 

 bildningssatt ur fysikalisk-kemisk synpunkt," Geol. Ftiren. i Stockholm Forhandl., vol. 

 xxxii (1910), p. 1506. On the formation of a eutectic zone in an orbicular hornblende- 

 gabbro, compare J. A. Bancroft, "Geology between Strait of Georgia, <£c," Geol. Surv. 

 Canada, Mem. 23(1913), p. 101. 



33 Op. cit. (10), p. 105. 



