part 4] PETROLOGY OF THE ARNAG-E DISTRICT. 483 



rhombic pyroxene, quartz, biotite, hornblende, plagioclase, mag- 

 netite, and disthene. The authors conclude (op. cit. p. 78) : 



' During 1 the intrusion of the Bushveld magma under locally exceptionally 

 active conditions, i. e., accompanied by marked tectonic disturbances, favour- 

 able opportunities were afforded of an interchange or transference of material 

 consisting essentially in a removal of alumina and addition of magnesia and 

 lime, whereby entangled masses of shale — xenoliths — were intensely metamor- 

 phosed, more or less permeated with igneous material, and converted into 

 rocks with highly abnormal mineralogical and chemical composition.' 



From this discussion of the two component parts of the con- 

 taminated zone, it is justifiable to deduce that there has been 

 a reciprocal reaction or interchange between the initial 

 magma and the xenoliths which results in a relative concentration 

 of, mainly, alumina and potash in the magma and of lime and 

 magnesia in the xenoliths. 



I wish to note here some of the observations of other workers 

 on the subject of reaction between xenolith or wall-rock and 

 magma that have come under my notice. 



Dr. Herbert H. Thomas x finds 



' the clearest evidence of the modification of a more or less pure aluminous 

 sediment by permeation of magmatic matter, more particularly by the 

 diffusion of lime, ferrous iron, and magnesia.' 



As already noted, A. L. Hall & C. Grardthausen (Joe. cit.) find 

 a removal of alumina and an addition of magnesia and lime 

 taking place in shale-xenoliths enclosed in the Bushveld norite. 

 The reactions which take place between the argillaceous enclaves 

 and an andesitic magma of Lipari described bj Alfred Bergeat ~ 

 are the same as those taking place between the argillaceous 

 enclaves of Arnage and the initial gabbro magma. The same 

 author 3 has shown a very important transfer of material at the 

 margin of the Concepcidn del Oro granodiorite. Emil Bergeat 4 

 demonstrates transfer at the contact of banatite and limestone at 

 Vasko. Reciprocal reaction is indicated by H. J. Johnston-Lavis 

 & J. W. Gregory 5 in their study of the ejected blocks of Monte 

 Somma. R. Brauns 6 finds a reciprocal reaction in the well-known 

 Laacher See rocks, and had stated many years before a belief 7 



1 ' On Certain Xenolithic Tertiary Minor Intrusions in the Island of Mull 

 (Argyllshire) ' Q. J. G. S. vol. Ixxviii (1922) p. 255. 



2 ' Der Cordieritandesit von Lipari, &c.' Neues Jahrb. Beilage-Band xxx 

 (1910) p. 575. 



* ' Der Granodiorit von Concepcion del Oro im Staate Zacatecas (Mexiko) 

 &c.' Neues Jahrb. Beilage-Band xxviii (1909) p. 421. 



4 ' Beobachtungen iiber den Diorit (Banatit) von Vask<"> im Banat, &c.' 

 Neues Jahrb. Beilage-Band xxx (1910) p. 549. 



3 ' Eozoonal Structure of the Ejected Blocks of Monte Somma ' Sci. Trans. 

 Eoy. Dubl. Soc. ser. 2, vol. v (1893-96) p. 259. 



6 ' Die Chemische Zusammensetzung Granatf uhrender Kristalliner Schiefer, 

 Cordieritgestein & Sanidinite aus dem Laacher-Seegebiet ' Neues Jahrb. 

 Beilage-Band xxxiv (1912) p. 85. 



' 'Chemische Mineralogie ' 1896, p. 313. 



