484 ME. H. H. BEAD ON THE [vol. lxxix, 



in the reciprocal action of enclave on magma and of magma on 

 enclave. 



With regard to the possibilities of diffusion in a magma, it may 

 he pointed out that Dr. N. L. Bowen 1 is of opinion that diffusion 

 through short distances is to be expected, and that reaction-rims 

 about foreign inclusions are readily to be attributed to diffusion. 



The abstraction of magnesia and lime from a gabbroic magma 

 recalls the opposite case supplied by the endomorphic changes in 

 granites by reaction with dolomites, described from the Pyrenees 

 and elsewhere in France by A. Lacroix and others. 3 



VIII. Conclusion. 



The main purpose of this paper is to establish the following 

 postulate: — the contamination process depends upon 

 reciprocal reaction between the gabbro magma and 

 argillaceous xenoliths; the magma becomes more acid, 

 the xenoliths more basic. 



I am compelled to leave for future discussion such important 

 points as these : — the significance of the Ardlethen granitic type 

 of product ; the existence and history of xenoliths complementary 

 to this granitic type ; the possible relation of such xenoliths to the 

 Arnage norite ; the operation of gravitative cleansing : and the 

 relation of contamination to petrogenesis. In the abstract 3 of 

 tliis paper, these points were briefly touched upon. The discussion 

 following the paper deals mainly with them. 



In conclusion, I wish to offer my thanks to Dr. J. S. Flett, who 

 has keenly followed the progress of this investigation, and has 

 been at all times ready with advice, discussion, and criticism. 

 I am indebted, too, to Dr. Flett for permitting five analyses of 

 Arnage rocks to be carried out in the Geological Survey Laboratory. 

 I also thank Mr. E. C Raciley, who performed these analyses and 

 many of the others used in this paper. A small party — Prof. li. 

 A. Daly, Prof. C. F. Kolderup, Prof. A. W. Gribb, Dr." J. S. Flett, 

 and Dr, Robert Campbell — has seen the field evidence with me, 

 and I have derived much advantage from the many discussions 

 which took place then. Finally, to my friends of the Scottish 

 Geological Survey, and especialty to Mr. M. Macgregor and Mr. J. 

 Phemister, I offer the expression of my sincere gratitude for 

 many helpful criticisms and suggestions. 



1 'Diffusion in Silicate Melts ' Journal of Geology, vol. xxix (1921) p. 316. 

 See also, Kurd Endell, ' Ueber Diffusionserscheinungen in Silikatschmelzen 

 bei Hoheren Temperaturen ' Neues Jahrb. vol. ii (1913) p. 129. 



2 See especially A. Lacroix, Bull. Carte Geol. France, vol. x (1S9S-1S99) 

 p. 241 and ibid. vol. xi (1899-1900) p. 50 ; A. Michel-Levy, ibid. vol. xviii 

 (1907-1908) p. 193 ; M. Longchambon, ibid. vol. xxi (1910-11) p. 323 ; E. Wein- 

 schenk, ' Vergleichende Studien iiber den Contact-Metaraorphismus ' Zeitschr. 

 Deutsch. Geol. Gesellsch. vol. liv (1902), especially pp. 459-61. 



3 Abstracts of the Proceedings of the Geological Society of London, 

 No. 1100, March 23rd, 1923, pp. 63, 64. 



